I had a similar situation at work a couple years ago

What's worse was that the woman's ex hu*****and was also a collegue.

I remember having this 2 "compartmentlized" feelings over the whole thing. I felt sad and hurt for their exes, particularly I knew the exhu*****and also. What happened to them were horrible. But I know I wasn't in a position to judge what they did and who they chose to live their life with.
So, when she invited me to an happy hour where the new guy was also there, I told them that they looked good together (which is true) and they looked very happy and I was happy for them. I could tell they appreciated it very much. We became closer after that.

That's just me.

As far as the rumor goes, they are each getting a divorce to be together for God's sake, do you really think they care about whether other co-workers are talking about them? Too late for that worries.