Internaldisease

Hello! Please bear with me as I am in my early 20s and figuring this all out by myself since my early 70s father wants nothing to do with this.

My father bought a house in FULL a few years ago and put his girlfriend’s (at the time) name as a co owner. Which is terrible, I know. They soon broke up and he moved out. She has been living there and paying the property taxes/utilities. She recently asked him to pay this year for the property taxes and fix the roof. He declined. He is willing to sell but I doubt she would be since that is her home and she is very emotional/hardheaded. What are the options if it stays frozen like this and one of them passes first or the next step of legal action?