You can have your own boundaries any time you want.
"At this time, I prefer you not have her over here because Christmas is a family thing and I prefer outside relationships not mix up with the son and family things. This is a limit."
Or something to that effect but in your own words. In the long haul whether it is a hard limit (never will change) or a soft limit (could change over time) -- you have to decided that. But hard or soft -- a limit at this time is still a limit at this time.
You can only choose your own behavior and manage your own stuff. You cannot choose his behavior for him. Let him deal with his things.
Having your own limits about what you are willing or not willing to participate in or deal with at this time is not preventing him from anything or you being a "gatekeeper bitch." Just state your position at this time on "Can I invite my gf to this xmas?" and let him deal with the rest.
- He can choose to tell you he wants to renegotiate the line with you and you engage in that kind of talk for future holiday plans.
- He can choose not to invite GF to the family shindig and make a separate xmas fun time with her on another date.
- He can choose ______? (something I can't think of -- he's his own person and can think out things for his choices.)
And if not, how can I not be that "bad guy?" What I mean is he is (essentially) saying to her, "I'm not going to tell my folks nor son about what and I am not going to ask what you want...but my wife doesn't want you over so my behavior doesn't matter>"
On that bit? You could chose to KNOW you are not a bad person just for having reasonable preferences and boundaries of your own. I wouldn't want a potential coming around a kid too soon. Shoot, in my world I don't want to deal in that at ALL. Hence CLOSED during parenting time. Everyone is different.
You could choose to reach out to her and tell her your willingness to talk direct rather than via the hinge, and your willingness to talk in trio for clear communication. You could choose to tell him as well. Or email them both at once. However method you choose -- you could choose to do that.
There. You are willing to talk. Neither of them take you up on it so you can sort things out as they come up sanely? Not your problem if they do not avail themselves of the reasonable opportunity given. *shrug* You can only control you. Not other people.