If the birf was limited to going in flush with the outer edge of the drive flange you are probably ok for the limited number of miles traveled. I'm kinda surprised that you didn't take the guy's head off. :G especially since you are such a nice guy. :ugh: It was certainly a stupid thing to do. They should have downed it til they got a snapring(IMHO). I would make the service manager aware of the issue in case there is a problem down stream so that you can say "hey remember when the idiot that worked on my truck the last time put it togther wrong....."
The snapring would limit inward travel by about .5" more than a bolt w/ a big washer on it on the end.  efinately get this one on the record with the manager for potential problems later on. Use a not so harmful mistake to your adavntage next time you cause some major damage that you need fixed. :G