It is more common than one thinks. I bump into them all the time. In fact many of the airport bums that hang around airplanes and airfields do a lot of talking and very little flying. Some of them due to financial or medical reasons - and I obviously have zero problem with that - but a lot of them because they lack confidence. Not that I have any problem with that either, unless they act like they fly a million hours a year when I know they don't. And this doesn't just concern inexperienced pilots either, I've had an instructor that did pretty much anything he could to avoid actually having to go flying (and he was ex US Navy and super experienced).
I had a bout of it myself just after the first time I took my PPL. Years later when I re-did it, kind of the same thing happened again. I was never really in love with flying small singles, like the 152. I like the little thing and think it's a great aircraft, but I wouldn't say I felt sheltered from harm in it..
With my own aircraft things changed a lot, but I have to admit it took a few hours before everything fell into place and I started to trust it. In my experience, after a 100-200hrs of flying, the comfort level goes up. Best thing to do for your friend is to just get on with it - challenge himself to unfamiliar flying, go on longer trips, maybe take along another pilot (this can help tremendously) etc and he will find that his confidence with each accomplished task will soar - this I can promise.