The bank won't just release you from the mortgage you are jointly liable. so unless your ex is in a position to remortgage you are going to be tied in here.
If you were to assume that negative equity were 25k, she could just take your 25k, buy a new wardrobe and the bank would still pursue you for any shortfall. That said you also need the bank permission to sell if the price you can get is less than the outstanding balance owed to them.
I'll let the mortgage heads come in and illustrate the finer points.