If he's snapped teeth off something he's going to need to completely take apart the bike's engine and transmission, its not worth it if the bike isn't a classic or vintage, it would just be better and cheaper to buy a new engine/transmission and be safe knowing nothing else is wrong. The amount of seals and gaskets he is going to need to buy is going to cost alot.
The reason I say that is simple, if teeth sheer off of a gear they ususaly end up going places they shoulden't. Its highly likely they damaged other gears and since it is a bike and the engine and transmission share lubrication, the little bits of gear could have damaged the engine itself as well.
If you are lucky, you will only need to replace a gear or two, chances are you will have to replace the main or counter shaft since some of the teeth are on those directly in most bike transmissions.