Dear Abby is missing the point when he says file it under 'her problems'. Fine advice if it was just the sister at risk. But having co-signed a mortgage, the mother must now maintain the house and the payments in order to avoid a default and a possible 'joint and several' claim against her and her hubby. 

Sadly there's no shortage of parents who have either refused to kick their offspring out of the nest or, worse still, put their own finances in jeopardy by helping them buy their own nest. Having not taught their daughter accountability and respect, she is unable to demand it of others. 

As to what advice to give, that would depend on whether the parents - even at this late stage - are prepared to grow a spine. If the mother is also half-owner of the house, she could insist on selling it off. It's more likely though that she owns nothing but the debt, in which case I would predict this will end badly for everyone - except of course the scum boyfriend.