What you just wrote is a HUGE danger sign for it potentially becoming more than just friendship.
Here's what you wrote...read it carefully as though someone other than you wrote it.
Quote: I was/am so happy to have someone I can confide things to. He became a friend through work when things were really bad this summer. When my husband left. A friendship has just felt really good to have.
If (and I do say if) you are confiding anything about your M to this man, stop! You remember CSW right, remember his W got involved with someone that started out as a good friend. It just doesn't take much to cross that line.
As for taking people at what they say I have found there's not many of us out there who truly say what we mean, at least not in our intimate R's. My H can't seem to spit things out when something bothers him, so he covers it up with false bravado of "I don't care" or "whatever makes you happy".