This would definitely be a tough call. My first instinct would be to put him out of his misery but I've heard a lot of horror stories involving the Fish and Game when people were trying to do the "right" thing. It all depends on the warden, some can be cool and understanding and others are by the book thumpers.
I had a buddy who killed his first buck on opening morning when he was 15 years old some years back with his dad and sister. As they were going through the check station there was a rookie warden and an older veteran warden checking the game and tags. My buddy had killed his buck several miles in so they quartered it up and took all the meat out and of course the head.
When the rookie warden came up to them and started checking the deer and tags he asked them in an angry tone, "where are the testicles? They're supposed to be attached to each side of the quarter." Anyway the rookie warden went on and on and was threatening them with all kinds of things until the veteran warden walked over and said, "nice buck son, is this your first?" to which my friend replied "yes". He then turned to my buddy's dad and said, "you guys are just fine, you guys can take off. Have a good day."
I also had a run in with a warden in Strawberry once. I was by myself and was loading a decent 3x4 that I had killed an hour earlier into my truck. He pulls up like a bat of hell, jumps out and runs over to me and says, "let me check your tag." I said, "sure it's right here on the antler." He looks it over and says in an angry d-bag tone, "you cut the wrong date out, you cut the wrong date out." To which I replied, "no, I believe it is the 20th" and he says, "no it's not it's the 21st, now cut the correct date out or I'll cite you." So of course I complied. At that point I started questioning myself, geez maybe I am confused about what day it is. So when I get home the first place I head is to is a calendar and wouldn't you know it, that sonuva B was WRONG! I was pissed, well not too pissed because I had a nice buck in the back of my truck. I was thinking though he should get his facts straight before threatening someone, especially the way he did it. This was before cell phones mind you, at least ones that would tell the date otherwise I would've whipped out my phone and shoved it in his face.
Back to the story at hand. After thinking about this for a while I think I would finish cutting up the elk I just shot and see what the other bull does. If he is still there then I would head back to camp to see if someone wanted to put their tag on it. I have a hard time trusting warden's and I'm sure they wouldn't trust me either if I said I was just trying to do the "right" thing. I got enough to deal with without getting slammed with fines and my hunting rights taken away.