The whole idea of having a designated guy is so the team WINS. If he can't hack it and you can stay up front, you have no obligation to go back and help pull him around so he can get his beauty rest and then TRY for second, which is always a crapshoot.

You did the right thing, you saw a lock on second and went for it getting the highest placing for the team.

If this same scenario had happened very early in the race, then I would say maybe the right thing to would be to back in the pack and try, with the help of your team, to reel in the break. In that case you could argue that he is designated and he calls the shots, but at the end of the race, when you have a shot, give up to help a teamate that is out of gas? No.