He sounds extremely inappropriate. I think she should for the simple fact that, if you don't give in, he will do this to another woman. I hate to say it, but his behavior makes me think that he has already cheated on your mom. Not to mention it's HER DAUGHTER! He sounds like he has some serious issues and you shouldn't have to deal with this - neither should she.

She might be at a point where she is afraid to be alone and will put up with more just to have someone? I say that bc I know that's what my dad is doing with his current wife. He has basically said it flat out.

I agree that he sounds extremely creepy. I would definitely ask her to sit down and talk. If she gets defensive just give her some time to let it sink in. Or, write her a letter. It might be embarrassing for her having a talk like that with her daughter? A letter might give her a way to process it alone and really give it some thought without that overwhelming feeling of having you say it to her. Because she has chosen this man, she may feel guilty or like you are accusing her so I would be very delicate with her. I hope you can figure something out because this is not appropriate at all.