It’s not just some capitalism v art thing either. It’s communicating and promising and accountability.
To anyone reading, if you’ve got a business partner and you think he’s given you 50% of a business because he said it one night at dinner, saying you’re cofounders— get it on paper in terms you both agree to. Go halfsies on a lawyer who helps you both accurately match terms (copyright assignment is one of those things where particular wording, intent and employment arrangement are important.) If you have doubts, maybe watch The Social Network or read about Hoefler&Co. (https://en.m.wikipedia.org/wiki/Hoefler_%26_Co.)