Peggy
That's something you should be proud of. I have met enough people who cannot make that distinction. Maryanne seems to think that disagreeing with her means that you are the worst possible friend.
It is not only that. She thinks she has control over everything. No, not everything. She said that she knows that he can kill her family but then he would have to face her and she would only ever cease to breath when she decides that it is her last breath because she has more fight in her than anyone he's ever met.
She's irrational in her believes and she clearly has the wrong priorities for a mother. I do not expect my father to even consider giving his life for me but I do believe that his priority would be to keep me alive and not to eventually win. Not that I think she can win against my father.
You can talk to her but I am not sure she will listen. I do not want her or my father to start a war in this place and I cannot see it ending any other way if she continues to behave and think like this.