I really don't think the entire thing is a "scam" in the traditional sense of the word. I think the conditions are probably mostly (or at least somewhat) accurate. There are two adults trying to live on a single SS check, a thing that was meant as a supplement, not a salary, and designed to feed one person (the one who earned it).
Where the bulk of the bullshit comes in is his 1001 excuses for why he's disabled and can't work. He's a lazy, leeching bum who doesn't give a single damn about his mother and has been rejected for all of his SSI applications. The mere sight of a job application makes him revolt. He's also stupid, which is making things worse. He even lets a lot of stuff slip that he shouldn't. I mean, this guy is so damned lazy, he won't even ride a bike to the grocery store, he insists it be delivered through a service. There are all kinds of red flags all over the place. And you are probably right, his mother probably encourages this. She's the one that raised this oxygen waster after all. I'm also guessing he/they squandered everything his late father worked for, and now they are broke with a 3500 dollar mortgage on what is basically a duplex.
That last thread may bring some change to the situation. Because if it's true, a few of the DUmmies were threatening to call the authorities to do a welfare check on the situation.