Why did he feel he couldn't mention a female coworker was going on the trip?
1) Because he is prone to lying by omission?
Or
2) Because you tend to grill him needlessly about things and make him feel guilty for stuff no matter what so he didn't want the headache?
Neither scenario would make it OK. He should have spelled out clearly who was going when you point blank asked him if he knew anyone else because we're all adults here and he should have nothing to hide. As an adult he should know it would be WAY WORSE for you to hear she was going when he pretended not to know or whatever.
But I would be MORE concerned about this if the answer was 1. Because deceitful people are more likely to misbehave. And if you haven't made his life difficult in the past for being honest he has no excuse.
If you HAVE given him trouble before for stuff even though his record is squeaky clean, then I'd be less worried about the trip, and work on the big picture.
It is a VERY BAD sign that he likes this coworker and has now lied to hide her presence. Again, maybe he's not cheating, but this is going in a bad direction for your relationship even if it just causes you to get all crazy (rightfully so but self-destructive) so find the root of the matter and work on it!
My ex was a huge liar by omission and huge cheater btw. The more accepting, open, and receptive I got as per therapy's advice, the more he lied because he just lies lies lies. But I still trust men who are clear and honest up front about things. Lying by omission on something like this? I'm way too scarred to ever work through that again with anyone. But if your relationship is good, you can figure this out! Good luck!
**As for name slip-up. Meh. Could be innocent slip. Especially if he's being deceptive and hiding her presence on trip, yet he's spending time with her working, so her name is in his mind. That makes him more likely to make blunders than if everything was all out in the open..again..work this out. It may be nothing but his needing to man up and be more honest about stuff.