In general, if anyone wants to get his computer fixed (at Best Buy or anywhere elese) without having his data stolen and risking identity theft, its best to look over the service persons shoulders while hes doing his thing. Be very suspicious if he whips out a USB drive and starts copying your files to it.
I had some computer problems a few years ago. The husband of a coworker was a computer technician. He fixed my computer for me but seemed a little annoyed when I picked up my machine. Now I guess it was because I’m boring!
If he did it gratis, I suspect he was annoyed because he felt you were imposing on him. Fixing computers is time-intensive and frustrating. It is doubly so when done without pay.