Getting Mad At A Guy For Starting A Streak With Another Gril

This girl I go to school with, told me today that her and the guy she was "talking to" were in a fight. When I asked why she said "He started a streak with Sara." (Sara is one of her friends) I told her she was being ridiculous because they weren't even dating, so he can still talk to whoever he wants. She got mad at me and told me I wouldn't understand. What do you guys think, is she overreacting about this or is she right to feel that way?