Divorce of older couple?

My mom (age 73) married a man (73) she only had courted a few weeks. To me, she just seemed to be slipping in her mental capabilities, not just because of this unwise marriage, but there were subtle hints. She's been married 1 full year now and realizes what a big mistake she has made. She is miserable with him. The man has some anger and emotional issues which are serious. He has recently made a sexual advance towards my 21-year-old daughter (mom's granddaughter) and I haven't told mom yet. I'm waiting until I can help mom with an exit strategy before I tell her. They just bought a house with $20k down - - which is ALL of mom's retirement money. If they divorce, is he entitled to 1/2 that equity since my mom didn't do a pre-nup? She cannot afford to pay him half and with the housing market, if they try to sell the house, they will probably lose any invested money in it. (he has none of his money invested in it) She is still showing early signs of dementia. What advice can you give towards this situation?