“Don’t worry if his family thinks you’re a monster for leaving him when he has cancer” might seem flippant or dismissive of the very real fear of being seen as a bad person.

I would like to tell the LW that she gets to not care what BF's relatives who facilitated his infidelity think of her. She is allowed to put their feelings into the box of "they get to feel what they feel far the fuck away from LW".

I would also advise LW to reframe the narrative with her own friends: He pressured her to stay. She can't trust whether he's being truthful about still loving her or just doesn't want to do cancer alone. She can't be his free 24 hour home health aid. She knows she's ~not supposed to say it~ but she didn't sign up for this.

Maybe throw in an, "It's like a switch turned off after I found out he'd had an affair and I can't turn it back on again. I don't love him and I'm afraid I'll be worse than useless, or even dangerous, to his health if I stay."

And I wish the advice had been clearer about reminding her friends that they're her friends first and she needs them to be fully and unquestionably on her side.