Well, we already know how much he's exaggerated his bullying, so I'm betting that's also a lie. The truth is probably he was harassing Kristin at school, she was trying unsuccessfully trying to hide from him, so her friends confronted Peter (and probably not for the first time) and flat out said to stop stalking Kristin.
If they told him that if he went to prom he'd ruin their good time, chances are this girl was very upset, they were fed up, and knew Peter well enough that if he showed up and started howling about how he deserves her as his girlfriend, yes, it would ruin everyone's big night. Because yeah, he's not the type to just be content attending without a date and to just hang out with people and dance alone (even though at my prom people with dates was the minority and loads of people just danced as part of the crowd). Proof of this is that he's crying about it almost 30 years later.