Brother (29) has a narcissistic girlfriend of 2 years and they live (rent) together. I believe she's been diagnosed with borderline personality disorder.
He's unhappy and exhausted and has wanted to end it for ages, but he's scared of her, to put it bluntly. He thinks she could kick off massively and also doesn't want to 'deal with her sad emotions.' He also feels guilty and sorry for her because she has no friends and a poor relationship with her family, so she has no other support. She's completely dependent on him.
Today he says he can't end the relationship and just hopes it fizzles out if he just gets on with his own life, and they start sleeping in separate rooms etc. He's now asked me to end it for him instead, and says he'll then move back with our parents.
I think this is insane, I'm not ending an adult man's relationship for him (for context he's the youngest sibling and used to everyone else in the family doing difficult things/taking responsibilities for him, he's quite immature).
Aibu to say no and tell him he has to grow up and end it? I think she's more likely to kick off (and rightly so) if I got involved! I'd be livid if DP's brother came over and ended our relationship.