My question involves collection proceedings in the State of: California

My parents are divorced, and have been for well over 10 years now. However they are both still listed on the mortgage for the house. A stipulation of the divorce was to keep the house until all the kids finished college and/or moved out and became independent. But as a result of the weak economy the house has been a rotating door of the kids moving back in. Beyond that they both became complacent with the living arrangements and life just continued.

The situation I'm trying to help my dad with is that he just received a notice in the mail that he is being held responsible for delinquent credit card debt that my mother incurred on her own. The debt is somewhere around $13k and they're tacking on about $7k in penalties and fees. The notice implied that he needed to settle the debt for her or the house could be put up for 'short sale' and sold for considerably less than the value.

So I really want to know is can the collections companies make that claim? Is our home at risk of being taken or having a lien placed against it? What can my father do to protect the house from these collectors?

Any advice is appreciated.

Thank you,
Kooie