Sigh. All I can say here is that you should never, never involve more than pocket money without a written contract. Even discarding malice, there may always be a misunderstanding, and a written contract makes sure there is no room for misinterpretation, changing ideas or bad memory on anyone's side.

As for this case, it seems to be an issue of reevaluating your relationship to this person at least as much as one of legal advice. So since you're asking what I would do, I'm pointing out the following which is heavily painted by my own personal bias.

The two points I'd think about here are that I take trust and respect very seriously myself and don't tend to forgive violations towards me either; and that I believe ideas are a dime a dozen; the dirty work of proper implementation is worth as much as the idea itself, if not more.

So I can see where you friend is coming from; on the other hand, s/he has made a promise.

You're already paying a royalty; that covers your side as far as I'm concerned, as that's what I would propose to be fair to someone who did my dirty work if I don't grant them rights to it. I would propose a higher percentage, but 15% sounds pretty significant to me.

Talk about this to him/her. Remind them they're getting quite a fair share. You could offer them a license to redistribute your property if you feel comfortable with that - not for free, of course. If they are at all reasonable, s/he should agree. If it works out, GET A WRITTEN CONTRACT for crying out loud. (Sorry for yelling. It really is extremly important.)

If they insist on their side, I say they consciously changed course on you. Personally, I wouldn't do that to someone I consider a friend (or anyone else, for that matter, barring extreme circumstances) - a clear case of disrespect and abuse of trust. In that case I'd follow the other Anonymonk's advice and secure what I can in restrospect. (And still get a written contract about as much as I can.)

Be fair - to them, but to yourself as well. Try to convey your perspective and come to a friendly agreement; but don't hesitate to take whatever measures available if the other party is not playing fair.

Makeshifts last the longest.