So many questions about money.stackexchange.com/q/123388/34875 that aren't related to the stimulus, but more about the OP's life circumstances...

Why are they not officially divorced? They really ought to make sure he's not filing jointly and forging their signature.

Why have they not filed since 2012? They might be passing up some credits they could be eligible for.

Is the disqualification from SSD/SSI permanent, or is there a chance they could become eligible in the future?

And the big one, what is their long-term plan for supporting themselves? Especially after their father passes away, which will happen at some point.