hmmm...so 'he didn't think there would be a problem' Time to call his bluff, I think.
How about saying 'it's so good you feel all that affair rubbish is in the past and we have all moved on. This woman, she must be a really super person for you to be so friendly with her so I'd like to meet her sometime, now that we can all be adults about it. I may drop by some lunchtime or perhaps meet up with you if you go to that hotel again for a meal, that would be nice, wouldn't it, after all there's no problem now is there....?
If you take this line, make sure you don't tie youself down to a definite meeting time, if your dh suggests one. See how he reacts to the idea of you dropping by with no notice. If he seems very uncomfortable with this idea, this is a danger sign. Ask him why!!
Like spidermama, I too was in a relationship where my partner was seeing another woman. I couldn't stand it and realised the only way to get at the truth, and feel I had some power again, was to meet her. It worked. However, that was a very different situation to yours. The other woman was not my husband's line manager. I have lots of sympathy for you, as this is a horrible complication.
I think you should come up with some concrete suggestions about how your husband must avoid this woman at work. And make sure he starts looking for another job nearby.
How about suggesting he goes to personnel and asks if there are any other hotels where he can get contract rates when he goes away? I am sure he can come up with a suitable reason why he does not want to be in the same hotel as some of his colleagues. And if he does stay somewhere different, you can ring to check - and ring the other hotel to check where this woman is staying. Still not absolutely watertight, but something at least.