Sounds like they need a lawyer to sue the mortgage broker. This sounds like an all-to-familiar situation these days. Buyers were put into houses they couldn't really afford using falsified documents. If they could have afforded them, why falsify the documents. Values fell, leaving them unable to refi. The mortgage broker, meanwhile, pocketed a nice payday. Maybe I'm reading too much into what your writing.
The solution is to move to a place they can afford, sell the property, paying off what they can and either getting the lender to forgive the rest or taking back a note from the lender for the deficiency.
Is there any chance the credit union would sell the notes at a significant discount? I'm thinking something like 50 cents on the dollar.
I'm not very sympathetic to the credit union. They may be crying fraud now, but they weren't paying attention when they made the loan.