What the others have said.
It is a good idea to get things in writing.
Recently an AW writer tried collaboration with an on-line writing partner. The partner--once their story got the attention of a small e-publisher--cut her collaborator out of the communication loop and presented herself as the sole writer of the story. This done and being batshit insane, she then tried to re-establish herself with the collaborator as her "friend." (Yikes.) There being nothing in writing, the AW member was flat out of luck.
I've co-authored three novels with a non-writer (he was a fast learner) and what made it work was mutual respect for each other's work and tossing the word "no" out of the room.
I went in with no idea if he could write (he pleasantly surprised me) and with a great respect for his acting talent.
He came in with respect for the fact that I'd (then) sold a dozen novels and had a clue about this craft.
And it helped that we liked each other and were able to communicate ideas rather well. I'd had some drama experience, so that helped.
But most important-- we left the word "no" out of the room.
It's something I learned from Chuck Jones.
When you know that none of your ideas will be arbitrarily shot down by the other saying "no, that won't work", it really frees you up to toss out ideas.
Not every idea is going to be good, but you never know that it might spark something for the other guy. I can't count the times when my partner tossed out something that my gut told me wouldn't fly, but then I'd put a spin on it and suddenly we had something to keep.
And vise versa!
On one occasion he came up with a scene that was very important to him, but I couldn't think how it could possibly fit into the book. It just sat there. I didn't say no, but I was discouraging, which upset him a bit. That in mind, I promised to see what I could do. Much to my surprise, the character took over and eased right into the scene and we all got away with it! THAT taught me the importance of not being negative.
Of course, careful about going in the other direction and saying "yes" to everything or you'll have a mess on your hands!
After banishing "no," the single most important way to stay friends is BE PROFESSIONAL.
I'm not sure how to explain that quality, but trust that you'll both have figured it out by the time you get together.
Good luck!