"Thin ice"? I doubt it. As long as this has nothing to do with his employers business, he's probably fine. Remember even if he's technically in "violation", the company has to care enough to go after him. Most of the time it won't be worth it.
It will matter when it matters. If he stumbles into making the next Minecraft, you better believe his former company will unleash the lawyers to at least get a cut or own the IP outright. If he ever seeks outside funding, investors will dig into IP ownership during due diligence and ask “when, precisely, did you start working on this?”
Every large tech company I worked for has been crystal clear during orientation: they own everything I do... on my own time or on theirs, using my own equipment or theirs. There was no ambiguity. Maybe his former employee is more chill. Maybe they won’t go after him. Maybe it won’t be worth it. That’s a lot of maybes to hang your livelihood off of.
If he stumbles on to the next Minecraft, he'll have enough cash to keep those lawyers busy for years while he still makes a nice living. Odds are he won't and nobody cares.
I was sued before over IP stuff, when I was a young kid and was working on a project. They only wanted to sue the large company with the big pockets, but I had to be named because I worked on the project. (freelance work)
Being sued really, really sucks, even if you have no real liability. Especially when you are young.
I settled for a pittance (a lot for a kid, but nothing for the lawsuit) and then they continued after the big player. So, just being in association with a larger player can be dangerous if you aren't protected well. (my lesson learned)
If he sat down with the lawyers and discussed the IP, he's a step ahead of the guy working on a game project, and not having consulted the legal aspects.
