I could use some feedback on a delicate situation.
My grandmother has diminished capacity from a stroke 18 months ago. She has varying memory issues, her motivation is impaired, and her emotional regulation is iffy, among other issues. According to the psychiatrist who recently evaluated her, she's intellectually and emotionally between 8 and 13, but with the fractured memories and habits of 70+ years. I'm a psychologist, and I agree; emotionally, she's deep in the surly, hormone-y teenager place; in terms of planning and focus, she and my 8 year old nephew are on par.
When she had her stroke, it was underestimated in terms of severity, because she didn't lose any speech, nor did she lose significant movement. (She now shuffles one foot.) Her social skills appear pretty hard-wired; she may hunt for words or circumlocute, or change the subject, or just echo what she's heard, but she can fake being fine pretty well, and if someone isn't paying attention, they won't notice her diminished capacity. But her personality changed -- she became impulsive, unmotivated, far more easily distracted, incapable of planning or concentration and can be easily manipulated.
The problem is that her stroke was misdiagnosed and she was on her own for about 6 months after it happened -- no therapy, no follow-ups -- and she lived then in a little town about spitting distance from the middle of nowhere. And sometime in that six months, she started a relationship with a guy I'll call Bruce who went to her high school.*
Bruce has about 5 years on her, and seems cognitively fine. He's also married, so their thing was an affair. (This is part of what I mean about her personality changed -- before the stroke, yes, she dated and had relationships, but adultery was not at all okay. "Don't mess with the married" is actual dating advice in my family.) He was sneaking out to visit my grandmother, made sure they were never seen together, parked his car where nobody could see it, uses a pre-paid cell phone to talk to her -- yep, all the markers of cheating.
I have zero objections to open relationships and consenting adults doing their thing. I have issues with cheating. The lies are corrosive, and they hurt everyone. I especially dislike being dragged into somebody else's closet to keep their secrets. I have very massive issues with consent. As I see it, there are three people in this relationship -- Bruce, Mrs. Bruce and my grandmother -- and only one of them has consented. Mrs. Bruce is being kept in the dark, and my grandmother has a permanent case of being rufie'd.
Currently, the relationship between Gran and Bruce is all by phone -- he's 1000 miles away -- but it was physical, and I think financial (there's money missing, but I can't figure out if Gran spent it, squirreled it, or gave it away). I don't think I'd object to the phone sexy times if Bruce was supportive and on the recovery team -- emotional peer support is critical. But he's not. (Or at least, not according to Gran. Who is, by definition, an unreliable narrator, but this is her emotional truth.) According to her, he tells her that she should return to their small town, that he will take care of her, that they'll be together. He tells her she doesn't need assisted living and that she can totally drive. (Again, this could be in Gran's head.) He appears to be the source of her persistent belief that she is only here temporarily and that she'll be going back Any Day Now. I've also heard some of their conversations and his messages -- I find the guy repellant, borderline abusive and a general jerk, but I'm a 3rd wave feminist who doesn't tolerate what women of my gran's generation took as normal.
That Going Back Any Day Now is a toxic belief for her, because it means she's not making friends, not interested in her community, not investing.** The truth is that she will never be independent again and Bruce will never be her caretaker. (I don't think it's a coincidence that his interest has increased as her assets have become more liquid.)
I'm not a parent, but this really feels like having a rebellious, infatuated 13 year old. (I remember BEING that kid.) The advantage to a 77 year old love-sick teenager is that she can't run away (literally) but the difficulty is that the tools I'd use with a teenager (block the phone, take it away, enroll her in an immersive activity) aren't available or are more questionable. She's an adult.
For me, the consent thing is the big issue -- Gran doesn't have the capacity to consent. If her relationship was with an intellectual peer, I would be less skeptical. If the relationship was long-standing, I'd be more okay with it. If her FWB was single, I'd be fine with it. But we're talking someone without an intellectual disability who arrived recently in her life, and is doing this clandestinely. Together, that's a lot of sketch.
I've got a couple of options. I can have a pointed convo with Bruce. I can simply block his number from calling her, and make him go away that way. Consciousness raising and logic hasn't worked (because this is dementia, not teenage hormones...) Or... ?
I'm just at wit's end on this, because I see the pattern: Bruce calls, they talk or whatever, for the next day, I get nothing but "When am I going home?" and she refuses to leave her room because he might call. She skips activities that she enjoys waiting for a call (which doesn't happen half the time). When he doesn't call for a few days (or she misses his calls because she's out), she gets more engaged in the community, but as soon as they talk, it's back to crap. Also, she tends to fall more when they're talking regularly, as if something he's saying is distracting or bothering her. (No, I can't just talk it out with her. She either evades or genuinely can't recall or confabulates, or doesn't want to disappoint me, or doesn't want to talk about her guy. It's the stroke.)
*They went to the same high school, but he graduated before she started, enlisted, married, lived elsewhere, then moved back a few years ago, and they didn't start hooking up until after her stroke. This isn't a life-long what-if-let's-try thing.
** That state has pretty wretched laws on nursing homes and assisted living. The law here is more elder friendly (greater latitude for what requires long-term care, no restraints, no alarms allowed). She's improved very much under the care she gets here, but she's reached a plateau and this is as good as it gets, and this good is regularly falls down, can't manage her own meds, can't remember to eat, needs help dressing and bathing. There's a reason she's here.
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And I'm glad you have a grandmother who has meant so much to you -- though I know it must be so hard to see her deteriorate like this.
My grandmother was only 68 when we lost her, and I was devastated.
But I did want to say a couple of things:
1. I commend you for looking after your grandmother. It's obvious you care deeply about her well-being, and from what you said about her setting up her POA, she obviously recognized how wonderful you are.
2. Distraction is a powerful tool. 
Although those with dementia can definitely be childLIKE, they lack the potential of children. So discussing, arguing, reasoning, and so forth really don't work. Even if you can get agreement in the moment, their lack of short-term memory makes any real learning nearly impossible. But distraction works, at least in my experience. In fact, just a couple of weeks ago, when my mother started asking about when she could "go home" and why my long-deceased father hadn't visited, I quickly pointed to a large bruise on my arm and said, "Mom, look what I did to myself!" Her mothering and nursing skills immediately kicked in, and we were on to another topic. And I'll confess, I've even used "Look, a squirrel!" with varying degrees of success.
Good luck and all the best.