That happened to a friend of my husband. His new wife owed back child support so they kept her AND his. Her kids are teenagers and they have only been married a year.
> His new wife owed back child support so they kept her AND his.
File an injured spouse claim.
This is an easier read than the IRS version: https://www.hrblock.com/tax-center/irs/refunds-and-payments/tax-refund-offsets-injured-spouse-form/
If he's the high earner, it still may have made sense to file jointly for tax avoidance reasons. If they make the same, it was a poor finanical decision.
In any event, if the ex-husband has custody and she is paying support, that's a red flag.