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.
That’s what my husband said. He said if the woman is paying support and the ex has the kids, something is wrong somewhere. She won’t hold a job. She hops from one to the other. That would have been my red flag.