Get the guy back

I have an associate and she can act very bizarre sometimes like she will smack my butt, make fun of me, be disrespectful with my stuff, and constantly gives me the middle finger when I genuinely say good job. She hangs out with me at this summer pe camp I am doing. There was this one guy I really liked and I am almost positive he really liked me, and he used to sit with me at lunch and hang out with me. Unfortunately she started teasing him and being rude and pushy and now he doesn't want to hang out with me anymore. I told her to lay off on the teasing a bit and she was like "he can suck it up buttercup and he can handle it." We are on way to high school freshman year, and she is incredibly immature. I am talking about poor hygiene, poor effort when we do group projects, and she takes other peoples stuff and drinks out of peoples cups intentionally. I want to know how can I get this great guy back?