D.B.
I'd be a lot more pissed off that he just informed you of this, that he made his plans with her before discussing them with you. That's a red flag.
Why aren't you going along? Is this a 2-room suite with separate bedrooms and a sitting area in between? If each parent is in a separate room with their own child, I don't see that it's much different than having, say, connecting rooms or even different rooms (after all, the kids go to sleep early). But again, he made plans with someone else without even including you. So you have more to be upset about than the room itself.