Its not worth much not really more then the $50 I will give you for it!
If he really wants to sell it his best bet will be an auction...guns like that dont show up all the time and when they do people go nuts over them. I have a buddy that collects WW2 weapons and he said he was bidding on a model 12 at a local auction and he said the gun went for WAY more than it should have considering its condition.
also...just to be clear,my offer of $500 was in jest if I had the money I would offer $2k plus depending on its operating and visual condition.
just a bit of advise....there are A LOT of fake model 12 trench guns floating around so it may be a good idea to have your brother get the gun authenticated...someone who knows what they are doing can do this just by looking at photos. Id hate to see him unknowingly sell a fake trench gun to an eager bidder and then have to deal with the hassle later once it was found to be a fake.