Sounds like they used a piece of copper with a flare to clamp the hose. Flair is probably good, but maybe not. Hose is probably shot. Clamps? corroded? When in doubt tear it out. That is cheap stuff to replace yourself.
Chlorine bleach is your friend. It eats diesel and shower grunge and makes women return. Spray it everywhere the diesel may have seeped, but not on bare wood. Let is soak a day. Let it sit in the bilge a while. Maybe at 50:1 dilution.
Disconnect that shower. That is a terrible idea. Never never do that. Even a dedicated sump is a poor idea unless you have a cleaning crew visit daily.
Great news, cal 39 guy!... Check all the other hoses as they were probably installed at the same time and may soon suffer the same problem as the fuel return line. Glad you didn't have a tank problem.. cheers, Phil