Best way to ask for permission?

I want to go to a music fest this weekend that a friend has tickets to. She was originally going to go with her ex but now he's her ex and she has no one to go with. Now she's asked me and I really want to go but it involves tell my girlfriend and dealing with how she feels about it. It's not that she... show more I want to go to a music fest this weekend that a friend has tickets to. She was originally going to go with her ex but now he's her ex and she has no one to go with. Now she's asked me and I really want to go but it involves tell my girlfriend and dealing with how she feels about it. It's not that she doesn't like my friends she just doesn't like me being alone with her, which is why I'm not considering bringing her along if she's going to be moody or act weird. It'll be a two day event so I probably won't be able to do anything with her on those days. Should I tell her? Is there a best way to do it (like being vague)?
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