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…

No, it isn’t okay to say mean things because someone rejected you. A person has every right to reject someone. You can’t force someone to date you. The person can’t help their feelings, they can’t force themself to want to date you. Being rejected does not call for being mean to someone.