Since, you've already found a solution to your problem, I guess my reply is unnecessary. Still, this may happen again and if it does, I think it will be good for you to have heard this little story:
I have had the same thing happen to me on two separate and actually pretty recent occasions. Same piece too! In both cases, they wanted to play my piece for a recital at their respective schools. The two of them (let's call them Brandon and Brendan) aren't that far from me, so we visit each other from time to time. When the three of us were all together one day, they each decided to see who played the piece better at this point. As you can surmise, they both had made some significant changes to the piece in much the same manner as with your friend. I didn't really like that, but I didn't complain. I merely mentioned it to them over the course of the next week or so. When they failed to figure out what I was getting at, I was forced to be more clear with them (something I should have tried in the first place). Brandon is generally level-headed and understood me when I explained myself more directly. However, Brendan was less sensitive to my concerns and got annoyed with me after a while. Long story short, Brandon ended up performing the piece the way I intended it and Brendan opted not to perform it at all (can you believe he chose Schumann's Fantasy Dance over me???). In both cases, the friendship remained intact. Brendan may have been a bit miffed, but I think that was just the result of him being a naturally abrasive and stubborn person. We're still quite close, and he has since performed some of my other pieces (albeit only at small church/family social functions).
Minutiae aside, the point is that if things had gone more poorly than they did, then I believe it would have been an indicator that our relationship was of lesser quality than I had realized. But as it happens, good friends don't break their bond with you just for telling them how you feel about something like this. In any case, I think your solution was a pretty good one, given the circumstances. It's a more fair and accurate assessment of the nature of what he'll actually be performing to call it an 'adaptation'. But if it ever happens again, perhaps you will try a different method. Anyway, such is my tale and the counsel I would give.