I think you’ve already made your mistakes on this one.
If you have agreed to work for free, you should stick to your word and deliver the product as agreed.
A suspicion is just a suspicion after all, so use this experience as a way to detect if your friend is being deceitful.
Maybe add a credit for yourself and if he removes it or tries to downplay it, at least you’ll have learned not to work with him again.
Either way, take it as a lesson and move on.
