Defiance of Duskendale happened after the tourney at Lannisport where Aerys turned down Tywin's offer to marry Rhaegar to Cersei. Rhaegar was 17 at this tourney and already a skilled fighter. When the Defiance of Duskendale started, Aerys wanted to show that he can handle the situation without the aid of Tywin and he failed big time. Then the royal army under the command of Tywin (Hand of Aerys) came and Barristan saved the king.
What was Rhaegar doing during this mess? Why did not he step forward and risk his life to save his father the king? Did he hope that Aerys dies in the dungeon and he can marry Cersei? I have an opinion that Rhaegar would not object or even willingly marry Cersei.