The "break up" talk that Heather delivers to Melinda is rooted in the idea that Heather wishes to advance socially and Melinda does not. In Heather's mind, her desire to advance is hindered by the fact that Melinda is not socially accepted or socially desirable. When she dissolves her friendship, Heather makes this argument although not in so many words:
I think it’s time for us both to admit that we...just...are...very...different.
This helps to bring out the fact that the social setting in Melinda's high school is one where people use others as means to specific ends. IT used Melinda for his own desire, while the Marthas use Heather to consolidate their own power. Heather sees no end to which she can use Melinda, therefore her friendship has become expendable. While it does not savagely destroy Melinda, it does make her more aware of the reality that governs her life at the time.