The answer to your question is stated (albeit indirectly) in the section entitled "Lunch Doom." This is the fateful moment when Heather (Melinda's so-called "friend") betrays Melinda and ends their friendship. Here is their conversation from the novel:
Heather: "You have a reputation."
Me: "For what?"
Heather: "Look, you can't eat lunch with me anymore. I'm sorry. Oh, and don't eat those potato chips. They'll make you break out." (1-6-107)
Therefore, the answer to your question is as follows: The reason Heather gives as to why she can no longer eat lunch with Melinda is that Melinda has a bad reputation. Not only is Melinda considered depressed and moody, but also she is still cast aside for calling the cops at the party last summer. As a result, Heather cannot be seen with Melinda because the "Marthas" group would never accept Heather as a result.
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