make sure he is mature...I babysit my cousins all the time and when the older one was 5 almost 6 I let him shoot a bb gun he was and still is very responsible and listens well..the other one who is 5 now I wouldn't even T H I N K about letting him pick it up...at least not yet. Just make sure he knows the rules and when you say "stop" he must stop right then and there take his finger off the trigger and put the gun down always.
I'd start him out with unloaded just to see if he does follow all the safety rules then you can put pellets/bbs/bullets in the gun.
way to step up for your friend looks like he made the right choice with you!
"If guns kill people do pencils misspell words?"
"I do not agree with a word you say, Sir, but I will fight to the DEATH for your RIGHT to say it!"