Is it okay/normal to say mean things to someone's face because they rejected you?

This may seem like bait, but I'm asking a genuine question. This happened to my girlfriend where she rejected her former best friend (in a nice way, even saying they could still be friends) and later got insulted, told to kill herself if she was so depressed, and said she never wanted to see her again. Some people say this girl is mean, crazy even, and others say she had the right to say that because she was hurt and my girlfriend should have said yes even though her best friend was a lot less mature and younger than her. They were more like siblings than anything.