Personally, I think you should tell him, for a number of reasons, firstly - he has a right to know. Now, it could be that he already knows, it could be they have an open marriage, or that have come to an agreement to turn a blind eye to her relationship with your H, so you might not get the big explosion in her life you want, but that doesn't change the fact he shouldn't be in the dark (if he is). Remember, it's not a secret now, it's not just the people having the affair who know about it, you know, and I assume there's others who know, it's really really shitty when the spouce is one of hte last to find out.
Secondly, you are currently protecting your H from the concequences of his actions and letting his need to keep it a secret take priority. He has no right to expect this. Keeping it from the DCs is a similar thing. If the affair isn't publicly known about, do your in-laws, friends etc know that the reason he's not living in the family home anymore is because he was having an affair or do they think it's just a general "it's not working out between us?" if so, he might be getting lots of sympathy at being thrown out because you've decided you don't love him. Why shouldn't he be exposed as a cheat?
Thirdly, the OW shouldn't be protected from the concequences from her actions. If you threw your H out a year ago, it's pretty obvious OW has taken the decision she doesn't want to leave her DH, so if forced into a 'him or me' decision, she might well not pick your H. Right now, nothing has changed for her - if anything, your H living outside his marital home has probably just made the OW's life easier - how they can meet up just when she's able to get away, not having to arrange two lots of cover stories. Right now, you know her secret, and she's only able to continue in her lifestyle hoping a random stranger who hates her guts is loyal enough to her to keep her secret. Why do you think you owe this woman loyalty?
But as I said at the top, if you want revenge or a big drama, be prepared you'll probably not get it, if they have an open marriage, or if the DH just says "thank you" and quietly accepts the affair then you might feel a great anti-climax. Don't try to push it or push him. Just contact him (I wouldn't do face to face, as others have said, he could lash out at you) and let him know the facts. Don't make any comment about his W that might trigger him being defensive, don't be emotional, just state facts and then completely leave alone. You might never find out the results of this action, don't contact him again and don't get upset if they end up stronger than before.
Also be aware, your H will definately be angry at you for blowing it all up. He will try to make it your fault that her marriage/family is in trouble, not his and OW's for having the affair in the first place. (I might be tempted to ask the OW's DH to not say how he found out so that you don't have to face that.) Also be aware that if OW's DH throws her out, she might end up living with your H.