When you refer to “she” in this question, I assume you are asking about Madame Loisel.  If this is the case, she does not like to visit her friend from school because the friend is richer than Madame Loisel is.

Mathilde Loisel is the main character in “The Necklace.”  She is a beautiful woman who thinks that she was born to hold a higher station in life than she actually holds.  She was not born rich, and she married a middle class man, but she thinks that she deserves better.  She suffers, we are told, because she feels “herself born for every delicacy and luxury” but her husband cannot afford to give her these things.  Because she suffers from the lack of luxuries, she hates to do anything that will remind her of her plight.  This is why she does not like to visit her friend.  When she visited her friend,

she suffered so keenly when she returned home. She would weep whole days, with grief, regret, despair, and misery.

Her friend was rich and visiting the friend reminded her that she herself could not have all the things her friend has.  Therefore, she does not like visiting her friend.  We are told this in order to help us understand just how fixated Madame Loisel is on material goods and on the trappings of wealth.  This will help us understand her character, which is what drives the events of this story.