Not acceptable. I'll admit that I've been the girl in a very similar situation before with having plans to see an ex who I only saw as a friend and the plan being to stay in his room, whilst having a boyfriend. The difference is that I told my boyfriend and asked his opinion - if he had said he didn't want me to go, I wouldn't have done. I knew it was a bit of a dodgy situation and something I wouldn't be comfortable with the other way around, although it wasn't just going to be me and him - I was staying at his whilst some mutual friends were also in the area.
Anyway, my point is that honesty always comes first, and that her lying to you about it is incredibly suspicious. She might have been worried you'd say no, but that's irrelevant really - if you were her priority than your feelings on this and you saying no would have mattered more than not getting to go. Because I have been in this situation before I think you should be careful not to be too controlling with her and her friend, although obviously you have reason. I was grateful that my boyfriend okay-d it, because it showed he trusted me, but he was clear that he didn't want us in the same room together, which I agreed to - and I also texted him regularly to let him know I was thinking of him.
The problem here is your girlfriend lying, your girlfriend not taking your feelings into account, and your girlfriend not being willing to accept that there are boundaries in relationships. She can't expect you to trust her enough to okay it, if she does nothing to earn that trust in the first place. I'd give her hell to pay OP. You can't just do that in a relationship - your partner is your priority.