This
sounds good, but it doesn't really work, at least not in the way you think it will.
There's no way you can imagine yourself in an unknown future. You can not estimate how much you will regret one thing or the other, because you don't know the situation you will be in. All the variables affecting you (economy, health, family situation) are constantly changing. You're not going to picture yourself as a homeless alcoholic who somehow is still really glad he built that app once and would do all it all over again.
The first bad thing that'll probably happen after he quits his job is that his girlfriend will leave him. Not necessarily immediately, but it'll be a strain on the relationship. Even though she will not admit it (perhaps because she isn't even aware of it), she is probably dating him because he is a guy that has a decent job, not some dreamer with a wacky business idea (unless maybe that's how she got to know him). Breakups can be really tough on men, why can negatively affect work and motivation. Not being the guy with the decent job, it will be tougher to find a new partner as well.
The second bad thing that will happen is the realization that "being your own boss" and "working on the stuff that is important to you" versus "showing up" and "collecting a paycheck" always sounds better when you're doing the latter. Doing the former is actually a lot of stressful work and you can not tell how it works on you until you have done it.
Lastly, living with regrets is not such a big deal. Who doesn't live with regrets? Whatever you do, you can rely on your brain coming up with rationalizations on why this-and-that just wasn't meant to be.
Having said all that, with "five years of savings" (more like two years, am I right?), doing a sabbatical just to try it out should be in the cards. There may not be a need to quit the job, many companies offer this. If after six months to a year you aren't on the right track, it probably isn't working out, but you will have learned a lot about yourself.