It's a terrible idea.
Legally, it works. A joint account with 2 individuals is a 3 party contract; acting in concert all 3 parties can change that contract and let one person walk away. When banks inspected every check before payment they would find an item drawn by someone who was no longer an owner and send it back "unauthorized signature."
Now, we don't look at checks, we simply pay them if the MICR line is valid. The parties to the "friendly divorce" agreed that he could keep the account and she would be removed. Papers were signed. Maybe the paper even says the parties agree to hold the bank harmless for any errors in future payments!
Now, the divorce isn't friendly anymore. He's about $1800 behind in his child support. She's finds a blank check in her purse. Writes it for $1800 payable to herself and signs her name. Your bank is going to pay it.
He's going to be standing on your desk talking about your parents and wants the money back in the account now. That "hold harmless" thing? His attorney is going to say that he had no way of assessing the risks, but your bank knew exactly what the risks were and took advantage of him.
She did not commit forgery. He does owe her the money. If it's later than midnight of the banking day following the banking day of presentment, it's too late to send the check back. If your position is not clear, you are standing between the dog and the tree.
Close account A. Open account B. What was okay once is not okay now.
Thanks for the invitation to chime in; I started to do it a few minutes ago, but know that people don't really want a lecture when all they asked for was some help. Usually, however, I can't resist.