You find Kat and your friends soon enough, and upon noticing your sustained injuries, take you to a field hospital to have your wounds looked at. You comply.
You report to a hospital, and your arm manages to be saved. You learn later that Kat was found dead in No Man's Land, having bled out from multiple sustained injuries. Without your friend to your sorrow, you return to the front.
I think this is a writing mistake because that would mean he died while we were attacking the trenchline not after we got to the field hospital..
I survived and being the only one........