That's too bad. I have fond memories of my 5th grade math teacher taking me there many times for ice cream because she used that as a reward for whoever had the best grades that week, or something like that. Oh well.
I guess you might say there were red flags there, but he didn't do anything remotely weird, then or later. He was apparently just wanting to help out a kid who was interested in the same stuff he was. Nowadays I don't think anyone would dare.