Server hacks help!

Im on a server with some guy and he is hacking somehow, He is a nice fellow, Im just trying to find what he is using. he duplicated a power 5 punch 2 bow for me and I dont know how and he has so many diamonds its impossible, he gave us a chest of diamonds.... Please I would like to know how he is doing this. Ive asked him, He is a hard headed one. Its a bukkit server btw