Definitely. The cog should not come loose from skidding, or "over time." It wasn't installed correctly. The dude was either lying or he doesn't know what he's doing. I would not go back to that shop.

Here's the right way:

- Grease the threads for the cog and lockring (optional, but I always do it.)
- Thread on the cog by hand, making sure not to cross thread it.
- Tighten the cog. My preferred method is to lightly rotafix it on, because you can get it pretty tight with not much effort.
- Thread on the lockring by hand for the same reason as above.
- Tighten the lockring with an appropriate tool. (I use a hammer and flat bladed screwdriver because I don't have the right tool. But that's the Wrong Way so I can't recommend it.)