That's ok, but not quite enough.
If I were you, I would tell him/her the reasons for that. That's how you're going to prove him/her that you really appreciate his/her force and time.
If you tell him/her your reasons you're going to help him/her not to repeat the same mistakes as before and to be better teacher for the future students + you're going to keep your nice and friendly relationship and that's the most important :)
I hope I helped you.
Please, let me know if there's anything else I can help you.
Have a nice day.