Father-in-law stuck with condo fees from kids broken marriage

My son and his wife lived in a condo that I own. Last year, they had marital issues and he moved out. She stayed, refused to pay me anything, did not pay the association fees and trashed the place. I had no choice but to pay my mortgage payment, but I did not pay the association fees. So, they ended up evicting her. Now, they are asking me for eleven thousand dollars in association fees, attorney fees, and citations for her trashing the place and not completing various forms that they asked for. On top of the fact that I have received no rent for a year and paid the mortgage out of my pocket, they are asking for this outrageous amount of money. They are claiming that they paid about five thousand dollars in attorney fees, and the missed association fees are about thirty five hundred. Why should I pay all that money. Please help me. What can I do without spending more money for attorney fees.