You've said before that you thought Rylis was a very cautious person, right, Melkor?
Well, in her mind, anyone who's going to try something like what the Rebels are trying has to be either really good and hoping to get lucky, or really stupid and suicidal, and she's banking on it being the first. She doesn't care about the overall situation, she cares about not getting shot down, and as far as she's concerned, that's going to be the only outcome if she gets involved in this fight.
Maybe if our other Squadron Leader hadn't dropped out, or if we had the TIE squadrons that Vader took, then it'd be different. But as it stands? Not a chance in hell is Rylis going to go along with that plan. And I doubt Arvus will like or agree with it much either.