Like someone said before chances are the hub is shot from riding with the lockring loose and thus the cog free to spin on and off a couple threads, and probably stripped them. A good mechanic would catch this, an amateur mechanic would catch this. Sounds like your "mechanic" is dumb as sh*t and the store shouldn't be allow him to be working on peoples bikes if he is as inept as you made him sound. Go back and settle this with performance, you could have gotten seriously hurt or killed because the mechanic, employed by a large company nonetheless, was unqualified.