It can be really hard to step back and watch someone else do something, and it makes it harder if the person does not do it well. Often college is a weeding process - to give folks the chance to try things, and if they fail, well, they've learned that that's not where their skills lay. In my undergrad, I started as a freshman with 7 other freshman techies, and I was the only one to reach graduation.
Right now the only thing you can do is step back from the situation. Its hard - as hard as delegating, which is something else stage managers have a hard time with. It sound like, in your head, you've been considering this 'your show'. Listen to your friends if they just need to vent, but if the things that they're complaining about are serious, direct them, with their complaints, to someone higher up in the department. And keep watch, if she is brought up to SM another show, then maybe you can bring some of your observations to bear. But if she is, then I'd sit down with someone and ask what it is that she's doing that you're not, that she's being considered.
And, if nothing else, take notes and learn some things not to do. Sometimes just watching things go wrong can teach you a lot. At lot of my first inspirations as a SM, was seeing someone do the job badly and me vowing that I would never work like that person if given the chance. Start by making a list of her actions that you disagreed with, and then working out in your mind what you would do differently in the same situation.
-Centaura