As others have said, it's most likely a
Slide Fire Stock. From what I've read here, they take a little modification to work on a M&P 15-22.
Looks like fun, but I'm not about to plunk down $370 + tax for one.
If it isn't a slide fire stock and he modified his 15-22 to full auto without jumping through the BATFE hoops & taxes, that full auto 15-22 is a one way ticket to club fed.