You can release your source code without making it freely available for people to use. Not every open source app is GPL licensed.

Being open source doesn’t mean it’ll get stolen lol. People can copy the code, but unless they add anything significant, everyone will keep using the most famous one. Plus there are several licenses which can add restrictions regarding ‘look, contribute, but don’t copy without permission’ Vulnerabilities would be discovered much quickly, and people will even contribute to it. He’ll still have the right to sell his app on the Play Store… or keep a premium type plan like he already has.