Cheating ex's new girlfriend 😌

My most recent relationship ended with him sleeping with one of my closest friends, I wasn't too sad about it as I know not to get upset about people who treat you like that but it's pretty shit. Now I have an issue because I'm still in high school and I sit near the girl in a few of my lessons and I don't know wether to leave her alone or say something to her or what? Any advice would be appreciated but I'm not an aggressive person and I don't like standing up to people because im usually wrong. (FYI the girl has lost all but three of her friends because of what she's done. She had a boyfriend and obviously the boy was going out with me). Help?! X