Classy. What else was she supposed to do? It looks like she helped stop it toward the end, but if they were going this hard in an elevator at a prestigious event, then I doubt its the first time something like this happened.
It's very weird situation, she's between her husband and her sister who she's extremely close with, plus she's aware of surveillance camera. Also, it depends a lot on what actually happened. You can say she's cold, classy, basically whatever, because verything does make a sense when we do not know why they're fighting and what they're talking.