I don't like that my girlfriend and my bestfriend are becoming really good friends
Why don't you want them to become friends?
I care about my girlfriend, but I'm pissed off because my bestfriend didn't tell me.
If this is an issue of trust you might want to talk to your girlfriend about it. Trust issues like this can harm a relationships future.
I just want her to focus on me, unfriend my best friend on facebook.
It sounds like you are having some trust issues here with her. You really ought to sit down and have a conversation with her about this.