I did tell the OW's husband about the A. The way I did it was very calmly and emotionlessly. I explained the situation and shared the "facts" (I had cell phone records and sent a copy to him). In my personal situation I didn't say anything I didn't have proof of (like I didn't know if they were sleeping together, but I suspect they may have).
But whatever I was certain of I shared (i.e. stayed out late after a class they had together, went together overnight to Disneyland... I don't have proof here they had sex, but they both did go overnight so chances are likely something did happen...).
Whenever you do anything like this it's best to be calm and logical and not come across as a jealious raving lunatic (so they can't come back and just accuse you of being crazy!).
My reasons for informing the OW's husband was that I felt is was important he be aware of this threat to his marriage. I wished him well in reparing any damage and eventually building a strong marriage.
Although, in my situation (which is probably much different than yours) I did talk with OW first and told her she might want to tell her H about the A before I did. So she did get some advance warning that this was going to happen. I don't necessarily recommend this to you.... these situations can differ quite a bit.
But I did feel her husband had a right to know what was going on. I know if I were him, I'd want to know.
There is no arriving, ever. It is all a continual becoming.