Ex-wife is custodial parent and it appears she is committing FAFSA fraud by hiding assets

If she “gifted” the money to her boyfriend, or used it for something like paying off their house or his car, that’s not illegal or fraud.

If he chooses to not pay it back to her after D graduates, that’s also not fraud, because it had to be a legitimate gift to be on the up and up. That risk is on her, not you.