こんにちは。質問ありがとうございます。
「裏切者」はtraitorと言います。
「裏切る」という動詞はto betrayと言います。
例
あなたは私たちを裏切った。私は裏切者なんかの友達じゃない。
You betrayed us. I’m not friends with a traitor.
例
私たちの中に裏切者が隠れています。
There is a traitor hiding among us.
またの質問をお待ちしています。
Because you are referring to a 'friend' who has committed the betrayal, a more informal term used for this is 'backstabber'.It suggests that as soon as you turn away from somebody and trust them with your back, they stab you from behind.
A more formal term is 'traitor'.
People also refer to the biblical character of Judas, who, in The Bible turns against Jesus and betrays him. So when a friend betrays you, you may refer to them as 'Judas' without any religious beliefs.
There are times when someone you trust betrays you. When this happens, its a hurtful experience. There are some ways that you can express this. Here are some examples:-
1. A traitor.
"She is such a traitor, she told my boyfriend that I was cheating on him, and she is supposed to be my best friend."
2. A snitch.
"Janet is such a snitch, she told the professor that we were planning to stay away from his class."
3.Two-faced.
John is so two-faced, he pretends to be friendly to me, but he says bad things about me behind my back.
Did you hear about John?
Yeah I know, he is such a backstabber. I can't believe he would betray you like that.
I know, it was pretty hurtful. I never thought he would be a traitor.
People surprise you.
I hope that helps.
Have a great day.
Will
Those are just some examples of what you would call someone that betrays someone else.
When someone betrays someone they are usually called by the type of betrayal.
Examples
Cheater - Someone who acts dishonest usually used toward a mate who sees others behind someones back
Liar - someone who lies often
Thief - someone who takes something that is not theirs
Backstabber - sneaky to hurt others
Usually a betrayer does so for their own advantage.
Another thing you can call them is Selfish, meaning they only do things for their benefit not thinking of others.
A traitor is someone who breaks a confidence or pretends to have feelings and affection.
Similarly, a false friend is not sincere in some way. A 'two faced' person is someone who has two different points of view at the same time - it may seem illogical....it depends on the situation.
"John pretended to be Richard's friend so that he could steal cash from his wallet. He was so two-faced."
These are the two words that I would most commonly use to describe someone who betrays a friend or does something that goes against the good of his/her collective group.
Example sentences :
- I would have never thought that he was a traitor.
- That backstabber doesn't know what he has coming to him.
You can use any of the above two words to describe someone who has betrayed you.
Examples;
- He is a backstabber. He stole my girlfriend.
- She is a traitor. She exposed me.
アメリカ合衆国 
日本 
イギリス 
カナダ 
南アフリカ 
