Sophmore gf wants to go to prom with a guy at a different school

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Okay, so, here it is. I'm a Jr. My gf is a sophmore. We go to different schools. She wants to go to prom with her ex's friend who goes to a different school altogether. Idk what to think. I mean, I trust her, but I still don't like the idea of her going to prom with another guy. It'd be different if it was to her own school and it was with a friend or something. But it's to a different school with one of her ex's friends. She asked me if I was fine with it, and, I don't think I really am, but I feel kinda controlling telling her that I don't want her going to it. What do I do?