Business agreement among friends?

Hello!

I have a friend who came to me with validated users and a potential market. Additionally, he is bringing some design chops and industry contacts for continued user testing.

He is coming to me for 100% of the MVP, roadmap, etc for the web/mobile presence.

I'm slightly paranoid of getting TOO far without a solid understanding of who "owns" what. Is something like this typically a 50/50 thing? Is there a simple contract that should enforce that?