This kind of reminds me of my sister in laws wedding. Her wedding came...well somewhat earlier than expected and was sort of a rush ordeal. My mom and I decided a great part of our present would be to hire a photographer to document the day.
So on the wedding day, unknown to us, one of my sister in law's friends (you know one of the ones who thinks they are a photographer because they went out and bought an expensive DSLR and zoom lens) shows up and keept getting in the (paid) photographers way. She was even shooting over his shoulder when he was doing portraits of the bride, groom and wedding party. I kept trying to redirect the errant "photographer". In the end, I felt so bad that I went ahead and bought a bunch of prints and got his "cd" package on top of the fee we had already paid him.
If I were you, I might tell the client that their photographer friend is welcome to come and observe, but you don't need them getting in the way with another camera. That would get you off the hook of actually having to teach, and get rid of the problem of a 2nd photographer shooting over your shoulder and essentially "stealing" your work.