I'm 17. My girlfriend is part of a costuming dance group. Her "biggest fan" has asked her to do a costume with him where they're dating. I'm not okay with it. I've never even met the guy. Is this weird for her to want to do? Am I right to not want her to do it?
Yes, this is normal for her, you are wrong to feel like that.
The fact that she doesn't want you to tell her parents definitely sends a red flag. Also, the fact that HE was the one to ask her when he is 10 years older than her is not right. He shouldn't be thinking about her in that way, even if it just was "pretend" and they are "friends".