Since it was a total oversight, I would make a joke out of it and tell your friend. My guess he is going to ask you anyway what the professional found.
A friend and I were using my embroidery machine and having all sorts of problems, can't tell you how many times we rethreaded the machine, even replaced the bobbin several times. I was ready to take it to the dealer and then one last time we broke out the book went step by step reading it aloud and then realized there was one little thing in the bobbin area that we over looked. After that it worked fine - I would have been so embarrassed if I had taken it to my dealer and it was a threading the bobbin error, so I was glad to find it first.