Just say you need to move on. The longer you wait the worse it gets. Let them know without going into excruciating detail that it's better FOR BOTH OF YOU (be sure to emphasize the benefits of not relying on someone who doesn't want to be there). In the long run the client/friend will respect you more for being honest and saving them from someone who will do sub-par work.

And as someone who would similarly struggle with this type of problem, I can definitely say you should rehearse what you want to say beforehand and stick to that.

Best of luck!