How long has it been since the first actual communication with the parents over the fees?
Normally I'd say to involve admin at this point and give them all the proof that you'd been reaching out, but if the parents haven't gotten notification less than a month before graduation I would let the kid walk.

It's unfortunate and frustrating because the history you've told us tells us enough about them, but if they really did lose their job recently and just found out weeks before graduation about the fees then it's really out of your hands. It's something to learn for next year.