IMO, that's a lousy shop.
See if they still have the bolt on the floor somewhere. They should properly reinstall the timing cover. The cover is needed to keep dirt and other particles from getting in and lodging between the belt and the sprocket teeth. This will cause accelerated wear and depending on the particle size cause belt breakage.
If they can't find it just pick one up at the dealer and do it yourself and be done with it. However I do have doubts about how well they installed the belt and such.