If the complaint and duel is just, it's up to his fellow Nobles to object to rejecting it. He should lose standing in the Realm, but that is up to the players to do. I don't think we can construct a system where the game can tell which duels have a justified reason behind them and which ones make no sense and/or can be rejected for a good reason ('afraid to lose character' doesn't have to be the only reason for Nobles reject duels after all). Unless you have some sort of Vulgarity System, but even that might be difficult without players having the whole story behind it.
First Blood Duels were removed because they were used for the wrong reasons. They have been replaced by Training Matches (on Testing for now). If there is a good reason to duel but it's not worth your life, you can duel till surrender.