I'd say get something written down, with a date and signature. It doesn't have to be fancy, it can be 1-2 sentences only. It should make clear what assets/right

I'd say get something written down, with a date and signature. It doesn't have to be fancy, it can be 1-2 sentences only. It should make clear what assets/rights are given up and that he made the decision with prior thinking about it. I don't think he'd claim the existing text messages are fake later but might claim it was meant differently, under influence (drunk) or pressured. If the company is big in the future such a (former) founder can stirr up just enough hassle and doubt to look for some kind of payout, deserved or (more likely) not. I've been in a similar situation (a company wasn't incorporated yet) but I got a nice signed letter. That gave me confidence I own assets, e.g. the logo, even though the other project person paid for it with his own money. I guess that also something to watch out for: did your co-founder pay for anything that isn't owned by company yet (e.g. domain registration).