My questions are: why did the bank not start negotiations sooner before maturity or talk to the customer to discuss that, and second, once that conversation began, why didn't you just do a short term extension to get through the maturity period and not put you or the customer in the position to start with?
I am more than a little concerned that your lender wasn't concerned. From the info you provided, I am looking at the bank here and not your customer and wonder what your lender told the borrower, and if the borrower truly understood the situation since they saw it as post maturity issue and loan negotiation. I'm leaning in the borrower's favor here.
I don't repeat gossip, so listen closely...