Its not so black and white, Dick. It appears to me that she's having an EMOTIONAL affair with him which means she's on two branches right now. As I said, if she doesn't want to be with her SO then she would, actually SHOULD, have left him by now but, she hasn't. IMO.. that means exactly what I said in my first post.
OP: You'd be better off backing away from your "friend" and letting her make up her mind without persuading her any further with your wants. She's a two brancher.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion