I'm going to tell my gf she couldn't hang out with her friend
Why do you want to tell your girlfriend she can't hang out with her friend?
Her friend is a trap, why should she eat dinner with her.
I understand that you think your girlfriend's friend is a trap, but don't dictate who she can hang out with. If you continue to try to control her, she will likely be unhappy in the relationship.
Why can't I tell her who to hang out with?
Because it's her life and she's allowed to hang out with whomever she chooses as long as it's not hurting anyone. You should respect her personal life.