I was in a similar position. I purchased my house before we got together. I made sure I paid every single payment directly from my own money and account so he would have no claim on it.

This veers into relationship territory vs personal finance, but is part of the reason they are splitting after fostering + 20 years together because she wanted kids and he didn't? Because that is 1000% why I left mine after the same time frame. I paid a very expensive lawyer a lot of money to make sure I got out with all of my assets. (I did have a heart though. I rented him a place for a year and paid all the utilities so he had somewhere to go. After that it was on him to get a job and figure life out)

Anyhoo. The point is. Stepmom will likely pay an expensive lawyer to preserve as much of her wealth as she can. Dad is going to NEED a good lawyer if he hopes to come out of this in decent shape. She seems to have laid a good groundwork to protect her assets.