Well, the band discussed it during their performance in the bar, where the lead singer claimed he knew girls like her. They showed off and strutted their stuff but never actually did anything. Not to mention Needy overheard them talking and defended her friend by saying she was a virgin, which would be much better than sleeping with creeps like them.
He and the band are Too Dumb to Live . It's really that simple. Though they might not have read the fine text about what happens if you try the spell with a non-virgin. Otherwise squick or no they should have been hunting down some little six year old. What? Once you're making a deal with the devil that involves a human sacrifice I'm not buying that you have a whole lot in the way of morals.
Or Jennifer never really liked having Needy around and once she was possessed, she started showing it in certain ways such as breaking into her house, eating her food, and vomiting black spiky goop all over the floor for Needy to spend all night cleaning it up or sneaking into her room, telling her how she killed someone before she broke in Needy's house, or sadistically making out with her to say that she could be next or taking Needy's close friend and her boyfriend and eating them. You know, high school stuff.
Jennifer always was kind of toxic and insecure. She goes after Chip because she's the kind of girl would hurt her "best friend" precisely because she's not really a good friend. Under normal circumstances a girl like Jennifer would have sex with Chip, or steal him from Needy even though there are other boys that like her. Possessed by a demon she chooses to eat him even though she could eat any other boy.
Where does Needy go after killing Low Shoulder? She can't go home and most likely had no desire to return to the prison center she was held in.
She becomes a demonic anti-hero, travelling across country murdering douchebags that try to sacrifice women for fame.