The first thing to look at is the identity of the colleague he’s been chatting with. If it’s someone you know well enough and trust, then I don’t believe it’s worth prying too much into it. The only time you should confront him and raise the red flag is if he doesn’t inform you before the said rendezvous is.
If the person he’s chatting with is someone you’ve heard of but don’t properly know, you should confront him around the time of the rendezvous’ occurrence. For example, if the said rendezvous is in a week and he doesn’t hint at it at least a day before, you should ask him about it.
If it’s someone whom your husband has dated in the past, you should confront him then and there. You deserve to know why your husband plans to meet her ex.
In the end, what it comes down to is the trust between you and your other half. If you have a well developed relationship he would tell you himself before engaging in the activity. The bond you share with him is also an important factor. If he doesn’t tell you, it means that there are problems in your relationships that you both need to work on.