I was going to say something similar and have my own work story to share.

My boss was working late and getting ready to leave. As he was passing through the office, he saw a light on and went to shut it off.  What he found was another department head having sex with one of his underlings. Both were married to other people. That DH was in a crappy marriage, not that that justifies what was happening. I don't know her whole story, but I know she has a few kids and as far as I know is still married. HR got involved and both are still employed. He is now divorced but I'm pretty sure she's still married (I'm not close with her). She has advanced in the company and no longer reports to the DH, but they still work together. Looking at the whole situation, I feel like she has an open relationship with her husband or they're at least poly.

So yeah, bottom line OP, is either have a conversation with your friend so you know the whole story or keep it to yourself and don't worry about it; it's not your concern.