[SyneRyder]

Been there done that for the last 15 years, it's one of the ways I make my living :) I love product businesses, they're a great source of semi-passive income. The lifestyle of choosing your hours & where you work is bliss.

But the OP asked specifically about earning money to replace a full time income, and consulting is a faster route to money. Product businesses take time to build an audience (you do have an email list of prospective customers, right?) and you're looking for thousands of customers. Whereas with consulting, you only need to find a handful of clients. Charge your client $100/hour (really you should charge more, or charge weekly) and you'll be so far ahead of most App developers. Which is easy, since most apps earn no money and many get no traction at all.

You could always try both. Split half your time into consulting, the other half into doing an app. See which works better and gets more traction. Hopefully the consulting half becomes unnecessary as the apps take off and become a hit.

[montbonnot]

I agree with $100/hour for consulting, it's "easy" money. It depends on how you deal with micro-management. If you work as a consultant you'd be treated as an expensive external resource by your clients. Which is usually not the best place to be. On the other hand you have your corporate main job where your manager owns your life, which sucks. So he might end up going nuts. It all depends on how you deal with structures and bosses. If money is the main focus then yeah, Starbucks, consulting, any factory or freelancing is the right path for him. If he wants to use these 12h/week to learn something valuable and give himself the opportunity to not have to think about money anymore, then he should invest a couple of month in learning how to build an entire product from scratch, marketing, how to interact with his users, UX, business strategies, priorities, you know what I mean if you've done it already. He might not make money at all but he will acquire a lot of valuable knowledge. Knowledge = long term investment. It's like stock versus real estate.

[SyneRyder]

That's a good point. If you've already got a full-time job with a boss, adding a micro-managing client on top is a fast road to burnout & reduced quality of life. And the best way to learn how to build a product & market it & deal with support is to jump in and give it a try.