If a friend betrays, first you should ask them the reason. Maybe a mis-communication is growing between you two.
This person is called a betrayer.
A traitor
- a disloyal person who betrays or deserts his cause or religion or political party or friend etc.
No one betrays anyone
I think you should ditch your best friend and replace her with another person who won't betray your trust by dating your boyfriend. That's what you should do.
Just say what you said. That she isn't honoring your friendship.
Judas iscariot betrays Jesus
Someone who betrays their country is called a traitor
When someone betrays you, how do you react.
It actually depends on how they do it. If it's in a really bad situation, you should ask why they did it. Depending on the answer, you should think about if they really are a best friend. If it's not that bad, you might get over it. Think about it.
In Breaking Dawn, Irina betrays the Cullens by telling the Volturi about Renesmee, Edward and Bella's half-vampire daughter. She does this because in New Moon, Irina's mate, Laurent was going to kill Bella. Jacob and his werewolf pack intervene and kill Laurent first. Irina is still upset about this in Breaking Dawn, so she betrays them.
peter petigrew betrays the potters allong with mr.lovegood and mundungs fletcher