Since the contract does not include an automatic extension to allow the lender time to complete his role, your seller has two options. He could amend the contract to extend the closing date to allow the lender time to process the loan.
Another option is to consider the buyer in default. However, since time is not of the essence to the closing date, the buyer might argue that a short delay in closing is not a material breach of contract. This issue may end up in court.