Has he been able to get the thread out or is it still stuck in?
If it is still stuck in..... The broken bit of the banjo metal is soft. Do you have an star socket set/kit, find a bit that fits but you need to give it a hard push, then just undo the thread just being careful. The star bit acts as a dethreading tool. Some times it can also strip the thread in the barrel breech, so when he rebuilds it gas it up some where quite and listen.
Cool quick delivery