Dear Trying,
First, my heart goes out to you. I am responding to your post, not because I have any advice, just because I can speak from experience.
I have been in a similar situation...although I am the WS. I became involved in an EA with a coworker--much the same as you have stated here. For quite sometime, I didn't even realize what was happening. I thought my marriage was perfect and safe. I thought OM and I were just becoming good friends. We talked about everything--sometimes on cell phones--but more often in e-mails--very late into the night and of course at work. I honestly thought that's all it was--until we crossed some lines in an e-mail one night. It never turned truly physical other than sex talk in the e-mails. We continued for about a year. Now, things have stopped and I'm working on No Contact at work. Which is almost impossible--but am doing it in the way I address him and communicate with him--it is all business, nothing personal. It has to be that way or we will fall back into the same pit. I thought we could emerge as friends--but we can't, so it has to be this way.
I'm telling you this because perhaps your SO was in the same situation. It can happen quickly and you can kid yourself into thinking you are just friends--I still think my OM is in that kind of fog-but "just friends" don't do things secretly, e-mail late into the night, talk frequently on cell phones, flirt, share everything, etc.--that is something else--it is called an Affair.
But there is hope---I looked hard at my marriage to find why I strayed--I never would have seen myself doing something like this, it is way out of character. But when I took a hard look--I found that the OM filled an Emotional Need that wasn't being met at home. Now I in no way blame my dear husband for this..because I should have recognized this and communicated it to him. But, now I can see through the fog and am working hard on devoting my time and energy into making my marriage work.
So, yes--it was an awful thing to do--but there is hope. But remember, he needs to see this for what it is and you can help him do that with the help from MB resources.I encourage you to pray hard and forgive your SO--and focus on making your relationship work.
Blessings to you.