How was Melinda's ex best friend Rachel changed in high school?

Rachel used to be Melinda's best pal. Melinda thought she was always someone she could trust. Unfortunately, that turns out not to be the case. After Melinda's traumatic assault, Rachel turns against her, believing Melinda to be out to get everyone at the party into trouble. But this is largely a product of Rachel's ignorance; at this stage in the story she still doesn't know that Andy raped Melinda.
In due course, Rachel comes to realize just what kind of a slimeball Andy really is after he touches her inappropriately at the high school prom. Initially, she couldn't believe what Melinda told her about Andy, but now it all makes sense. Now that she's got the full picture, Rachel reaches out to Melinda, holding out the prospect that their broken friendship can be put back together.