I would get super excited! I'd try my best to stay calm, and then go outside, and ask her if it's really her XD
I'd try to have a bit of a short conversation with her, but she'd probably be in a hurry, so I'd try to keep it short. I'd just let her know that her music means a lot to me and thank her for bringing it to life. Then I'd tell her good luck, and I'm always cheering her on.
An photo would be nice so people would actually believe I'd met her, but not necessary, just being in her presence would be enough!
