I'm angry at my gf for drinking something she already swore off
Why is this something that makes you angry with your girlfriend?
Because I think it's selfish and immature not to respect other people's wishes and decisions
It makes sense that you would be upset with your friend for not being honest. Watching your friend back to a bad habit is probably very frustrating for you to watch. If this was something that you both agreed upon then they are also breaking your promise.
Actually it is not something I have decided upon with her. But ok I guess that might be something.
It is not reasonable to expect someone to keep a word when you never talked to them about promising anything.    You should communicate with them and express your concern before doing anything else.