A copy of his mom’s taxes showing that he is a dependent on them might be useful too as well as that danged proof that he is a non filer, perhaps other evidence that he is very low income with few assets. Is he on SSI?
Here’s the thing. There are cousins in our family whose father is pretty much in same situation as your ex. But the man, though has no income, is very well to do because of trusts and other family help. Though FAFSA and the IRS might not tag him with the funds, PROFILE might. That’s why these waivers can difficult to get.