I'm at a busy concert and I'm going to tell a girl that she should use the woman's restroom.
Is there a reason you think you should tell her that?
She's an inconsiderate cunt and her using the men's restroom causes a long line.
You shouldn't refer to people using derogatory terms. Furthermore, I feel like having a long line in the restroom can be very difficult especially if there is a lot of people, and the woman should use the restroom that is going to be more appropriate to her. However, that woman may also be a transsexual, and maybe more comfortable using the men's restroom, so I would think twice in recommending what restroom they would use especially if it would hurt their feelings.
I just want to tell her off.
It's understandable that you're frustrated, but telling her off is rude and it will cause an unnecessary scene. You could also hurt her feelings and make her feel uncomfortable and people might view you as an unkind person.