TBH I feel like there's very little risk with quitting to work on a project that already has users.

It seems like he's a good enough programmer that he could always find a job at a big tech company, make a bunch of money and then quit once he has enough saved up. And that's assuming he doesn't find any paid users or get any source of funding which again is hard but not impossible or that unlikely given that he already has a product with users.

I think he'll be just fine and I wish him the best.