In the first place, the owner never said what he paid for the gun, and no one has given an estimate of what they would charge to repair it, after a "hands on" check. Therefore, all the statements that repair is not worthwhile are completely un informed. Skeettx has given possible sources of hammers, and there are many small town gunsmiths that have old hammers, many from Belgian "hardware store guns", that may very well fit. I even have a can full, myself and don't claim to be a gunsmith. I, myself regularly shoot similar guns that started off seemingly as bad or worse( I haven't examined it either)than this one, and I consider them worth what I invested in them. I still say check it out before writing it off.
Mike
Thanks for your comments, Mike.
My son indeed had it checked by a local gunsmith yesterday. The gunsmith saw nothing to indicate the gun cannot be fired as-is beyond the need for the missing hammer.
Having said that, my son will likely not try and fire it, as he prefers to just save it as it is. He loves it.