What freaks you out? He makes $100 - $150 a day from app sales, which gives him a comfortable financial cushion. App sales put aside, he reckons that he can keep going for several months on savings alone. And he reckons that, should everything go wrong, he'll have no problem picking up contract work, which I have no problem to believe if he's even just a half-decent developer.

No big or crazy risk-taking here. It's always scary of course to go off on your own when you've spent your entire life in the comfortable employee universe so best of luck to the OP!