It was a bit late in the day, and cold in the shop, and he needed it done as he uses it for work, - hardwood flooring and that type of stuff. So I couldn't say 'I'll fix and get it back to you.'
The way I would have approached it is put the nail gun in a vise, take a look with a flashlight with what's wrong, find the right tool..... etc.
I don't even turn my back for 1 second and he's grabbing everything that is a wrench or could be used as a wrench and trying to disassemble it with 1 hand holding the nail gun and 1 hand holding a tool while trying to break loose some nuts. Scraping and slipping around on top of the workbench getting nowhere fast.
Turns out a nail had folded over and jammed the drive pin. When I show him how easy it is removing fasteners with the part held in the vise, he suddenly loves the vise. But he doesn't know right to tight left to loose with the vise handle. All the while I'm saying "slow down bud", "I think that's the wrong size, bud", "you don't want to do it that way, bud"
I resisted "are you out of the fuggin mind, A hole !!!!!" for the duration, and though he pushed my OCD to the limit, I cleaned up and worked on finding the humor in the situation afterwood.