Jester, if the questions are asked tactfully, where's the harm. If someone has an "I'm the boss and my word is law" attitude and teaching style, I'm not going to want to be treated that way and won't go back. I want to know this kind of thing before I agree to anyone teaching me anything. If I asked how his last student did and was told he didn't finish, just stopped showing up with no explanation, I'd be worried as well. Tact can make the most direct question look like friendly conversation if done right.