Exactly what the bank said: go to small claims court.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
You shall and can do nothing. This is your son's job to do something. He has or should have a lawyer. Perhaps his lawyer can help your son. The bank is not liable to your son's error and you have no authority to do anything.
And it may be enrichment, but not "unjust". Just because money winds up in what seems like the wrong place doesn't mean she wasn't entitled to it pending the divorce decree's being entered, especially if there are interim orders for child support and maintenance. Which raises a question in my mind: is your son handling his divorce by himself? Because these are things he should be having his divorce attorney deal with, not his dad. You're lucky the soon to be ex even spoke to you.
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