I can't see anything wrong with saying to your DH-I feel uncomfortable with this. I am not happy that you are choosing to spend family time and money taking a work colleague out for a meal in these circumstances. If that's how you feel.
No one can tell you if you are right or wrong to feel that way, some people seem to be happy with this situation, other people have different reasons for being unhappy with it.
But it's your relationship. You don't have to react the same as other people. You don't have to live your life according to someone else's feelings, do you?
If, for whatever reason, you are not happy with this, well that's ok.
Tbh, in the situation you described, I wouldn't be happy either.
But if you feel that by expressing this to your DH that will cause him to sneak about and hide what he is doing you must have some doubt about his honesty. And I think that is the issue, really. Not whether you are too jealous or whether you are unreasonable to ask your DH not to do this.
You should be able to express how you feel, have a discussion about it, resolve it. Not spend the next few months wondering if your DH has heard you say you are not happy but is out doing it anyways.