Don't say hi to her when you pass by her in the hall. Don't talk to her at all, that'll make it easier. Focus your attention on the guy standing next to her. Try to ignore how much it bugs you that his hand is around her waist. Don't you see his eyes show nothing but pure content, laughing whole-heartedly at something one of the dudes said. Of course he's happy. Don't stare. She laughs too, and it's the most beautiful sound my ears have ever heard. Don't let her get to you. Try not to loose yourself in those brilliant green eyes. Don't let her catch you staring, and don't ever let him catch you staring. Play it cool, act casual. “Isn't she great, bro?” give him a nod in agreement, tell him he's lucky to have her.
Don't expect her to turn around, because her hand is firmly intertwined with his. Just watch her leave, breath out a sigh, and wish things were different.