I had a similar situation once where I was doing work for friend of mine who is an MD and also happened to be my landlord. He never had to tell me where he'd done electrical work because I could just tell by the code violations I found.
Finally, I told him: "Take this in the spirit that I'm giving it: I would not try to practice medicine because I'm not trained in medicine, and I would make mistakes. By that same token, you should not practice electrical work."
Sometimes there aren't really many ways to tell someone something, and you just have to depend on them being humble enough to understand. Good luck.
-John
