Collaborating without a contract you regret
Most collaborations between friends go wrong at the same three points.
Two people make something together, it does well, and then it gets complicated. Almost always at one of three points.
One: nobody agreed the split
The conversation feels awkward while the work is going well and impossible once money is involved.
Have it at the start, when there is nothing at stake and everyone is generous. Write down the percentage. A message in a chat is enough — what matters is that it exists and is dated.
A split agreed before anyone knew whether it would work is almost always fairer than one negotiated after.
Two: nobody agreed who controls it
Separate from the money: who decides where it goes? If one person wants to license it for an advert and the other does not, what happens?
Decide whether either party can act alone or whether both must agree. Both-must-agree is slower and it prevents the worst outcomes.
Three: nobody agreed what happens if it stops
Collaborations end. Someone moves, someone gets busy, people fall out.
Decide in advance whether either of you can keep releasing the shared work, and whether the split survives. This feels pessimistic during the good part. It is the reason the good part ends cleanly rather than badly.
What this does not need to be
A lawyer, for most projects. A written agreement between two people that says what was decided, sent to both, is enormously better than nothing and takes ten minutes.
Get a lawyer when real money is involved. Get a message in a chat every other time.
The actual reason to do this
Not to protect yourself from your friend. To protect the friendship from the work.
Most of these disputes are not between dishonest people. They are between two people who each sincerely remember a different conversation.