Why does Greg call Maura in Andrew Clements's Lunch Money?

The only phone call made in Andrew Clements's The Lunch Money is actually made by Maura to Greg soon after they start fighting about Maura copying Greg by making her own comic book. Certainly, however, Greg does call Maura some names in the story.

In chapter 7, Greg storms into Mr. Z's math class, slams Maura's unicorn book down on her desk and calls her a thief, saying, "You stole my idea, and you know it" (56). He even calls her "cheap, stupid garbage" when he says her book is "cheap, stupid garbage—just like you" (57). It's during their fight that Maura accidentally hits Greg in the face, giving him a bloody nose and black eye. While waiting in the nurse's office, Greg has a chance to look over Maura's book and see that her artwork, which is original, is also really good. Later, Maura tells Greg she read one of his books, which she pays for with a quarter, and tells him she thinks his is good, too. During a conference with Mr. Z, which calms the children down somewhat but doesn't help resolve the issue, Greg gets another bloody nose and must miss math class. Later that same evening, Maura calls his house and offers to give Greg the details of the math assignment he missed. She next starts talking about his book once more, saying she looked it over again, and that his book "makes her unicorn story look just awful" (86). She then says she doesn't understand exactly why her book can't be considered a comic book, hoping Greg will give her some advice. Greg grows suspicious of her motives, however, thinking she is only calling because she hopes she can learn more about his ideas so she can steal from them, so Greg ends their phone conversation. It's not until much later in the story that Greg and Maura decide they'll be able to sell more comic books if they work on the book together.