Hmmm, I'm not sure any extra suspicion/demands for clarity are warranted. If hatingtoberight's account of events is accurate his only transgression is not telling her himself about what happened and I can certainly see why someone might choose to do that. It's forgiveable.
Rather than going over and over what happened as though he cheated (which it looks as though he didn't, even emotionally) I think you need to have the conversation morningpaper suggested:
now that her feelings are out in the open, and given that she is still pursuing him, clearly it is not appropriate for them to continue working so closely together. These are the moments when you need to put the health of your marriage first. It is not fair to you, things being what they now are, for you to have to kiss your husband goodbye every morning knowing he is going off to spend the day with a woman who is openly trying to have a sexual and romantic relationship with him.
Where can he move within the company that would limit his contact with her? What job opportunities are there in other companies locally in his field?
He also needs to break off all non-essential and non-business intercourse with her immediately. No CDs, no friendly e-mails, nothing.
Dior - I don't like being told that someone is attracted to me if it's not reciprocated. I find it embarrassing and awkward and where it's obvious that I don't or can't return their feelings I resent them for telling me and making things weird. but that's just me.