I wouldn’t say you are overreacting. I would be really hurt to know that her friends are bringing their partners along too. Totally understand if it was an all-girls event. I get you don’t want to ruin her birthday so I agree and that you should wait till afterwards. It’s something that she needs to know how hurt you are. One birthday something sour happened and my husband felt bad bringing it up and potentially ruining it. We talked about it afterwards and sorted it out like adults and overall I’m glad he brought it up. If your girlfriend really cares about your feelings she will hopefully understand.

I would feel a little upset tbh if this was my husband. They are coworkers and only need to talk about work related conversations. I understand a couple of “how are you?”, but your husband now needs to set boundaries.

Examples he could do it subtly is mentioning you with every personal comment. “I wish you could show me how to swing a bat” respond with “yeah, my wife and I like to go batting every Sunday” or even “yes, my father in-law really got me into baseball”. He should also not ask questions back. If it’s not about work, I don’t think he should really be talking to her on a personal level. At the end of the day, they are coworkers not friends and should really only talk about work things.