I am a broker and know some things about this. Call me at 215-852-4469 and I can talk to you about it.
First thing you need to do is secure every record you have regarding the loan in default.
Second thing you will need is an attorney who specializes in broker-lender contract law. I can recommend one if you shoot me an e-mail.
This is a serious issue. I have heard through the Grapevine of at least three brokers sued into BK over repurchase demands. Hopefully, this was a full doc loan...