Did your student us SOFT solder instead of EASY solder? Easy solder will pit if overheated since the zinc in the allow will volitize at high temperatures. Soft solder will easily pit the silver since the elements in the composition of the SOFT solder melt and volitize at a relatively low temperature. Check to see at what temp the solder he/she is using melts.
Thanks for the suggestion. I think you are probably right. There wasn't any of the solder left to run any tests on, but from your discription, I'd say that it most likely was soft instead of easy solder.