Rala killed a man in a duel and it has caused her a lot of grief so I think she would have good reason to want to intervene in a duel, especially one that is implied to be to the death. Glad hammerhand isn't gonna kill the kid though. Other than stopping them from killing each other, only other reason she might join in is if there is collateral damage. This is a squabble between those two, not the commoners