Ok lots of good answers from people who have been through divorces, so let me give you some advice from someone who worked at a bank. Get a lawyer, but even if the divorce decree indicates that she is responsible for the mortgage payment, it's not good enough. The loan note supercedes the divorce decree and they can still come after him for the payments, and every late payment she makes will still effect both of their credits. I have seen so many people come in waving a divorce decree, saying that they are no longer responsible for the car, house, credit card, you name it. It's a very rude awakening when they find out otherwise. A decent lawyer should be able to explain that, but you would be surprised how many don't
Sell or refinance are his only options here. The same goes with joint accounts. You only need one person to close it, so if he hasn't then he needs to. She can draw it as negative as she wants and your brother will still be held liable.