Your OP is a bit confusing.
Are you saying you were going to go to the theatre but your girlfriend realised the dates clashed with a local beer thing?
It sounds like beer thing is something for you all to go to as a couple of couples at least, if so then yeah, it's a bit odd if your friend has added his own girlfriend to the group chat to plan the beer thing if it's something your girlfriend is going to too, why not add everyone to group chat who is going?
Group planning when some of the group participants aren't even in the group chat can be stressful and take ages, because it needs someone to contact people who haven't been added to group chat and report back to the chat, also gives room for miscommunication when at least if all people going can see all the messages and are part of planning, less mix ups can happen.
Much easier to chose to have everyone attending in the same chat and she probably thinks because he chose not to add her, then him or his girlfriend don't want her to go, is there a reason you can't ask for her to be added? I can see her point of other girlfriends and partners are involved in the planning.
If your mates partner and your girlfriend aren't going to the beer thing, then none of them need to be in on the chat and I'd kind of think if my friend started a group chat to plan something and added her partner who wasn't coming, I'd find that odd too to be honest.
It's hard to tell what you meant in your OP though.