Now.

Depending on local laws, what little you have written down (eg emails), etc it may be fairly easy to demonstrate that this is a work for hire. It need not be anything said between the two of you - it could be an internal email demonstrating your understanding at the time. Alternately it may be possible to tape a phone call where you remind your friend of what was said before and get their acknowledgement that they said this and now want something different. That may be enough to prove a legal case. Don't let your friend know that you have taken this step, but take it.

After finding out what your legal position is, you need to negotiate a written contract. You would prefer to do that without getting overly hostile. A good lawyer likely knows more about how to get there than I do.

Oh, and good luck.