S.H.
I have been in a situation very similar, except I was the one with a friend at work. The situation started as just two friendly coworkers talking over cocoa. (Neither of us drank coffee but still demanded a break like the others.) We eventually got to talking about things going on in our personal lives and had great conversation. Let me say now that we never EVER slept together, but it could have easily gotten to that point. I did find myself wanting to spend more time with him and less with my husband. My husband was furious. I knew we weren't sleeping together so I didn't see where I was doing anything wrong. But it still almost ended my marriage.
My husband and I did go to marriage counselling and after 7 hard months, we managed to put things back on the right track. As part of the counselling, we were made to read a book called "His Needs, Her Needs" and one called "Bounderies in Marriage". They helped me realize what I was doing was wrong. But my husband telling me just made me more angry and rebellious.
My point is that there may truely not be anything going on, but if you are uncomfortable with the situation AND her husband is uncomfortable with the situation, then something should be done. Whether it is meeting these people with all 4 of you spending time together, or suggesting counselling to your husband if you can't get him to read the book. . . stop the situation now while you are still able to talk about it.
I wish you the best of luck.