I don't think that my non-binary friend is feminine enough to represent my gender on stage.
Why exactly is this a concern for you?
I don't have any problem with her, but she's just not great on stage.
You never know how people might preform without giving them a chance. You should support your friends when they try something new.
That's not how it works. She has to be good when she's onstage. I can't just stand there while she messes up.
If you've already seen her perform and her performance was lacking, then it's reasonable to not want her to mess up their production. Just make sure that you're not excluding her based on assumptions about her gender identity. Non-binary people deserve the same opportunities as anyone else.