How do I convice a girl I like that she's pretty?

So I've liked this girl for a while but her boyfriend is a manipulative and ugly prick who somehow convinced her that she was lucky to get him. Which pains me because she is a funny, smart, beautiful person and I just can't make her see it. I can't come right out and say "You're better than him" because then if I ask her out she'll think I was betraying her trust. I don't need to be too discrete, though, because I'm really good friends with her and I have a flirtatious personality anyway. I think I've done a good job of convincing her that she has an amazing personality but I'm having a tough time convincing her that she is pretty. Any tips? [This is high school btw]