Only thing I see is the ex husband still has control of the ex wife. Ex speaks, ex wife listens and gets upset.
Ex husband is shooting back for the upcoming new expense.
Sell the AR and use proceeds for the braces.
This is the best advice. The ATF or any other law enforcement agency won't care or get involved with who is the legal owner. You have broken no laws, so long as you are legally allowed to possess firearms. Sell it legally and you are done with it. Don't admit in writing that the rifle is his or that you are willing to send it to him. He's not going to do anything but trying to tick you off. Keep documentation of the sale and what you received for it.
