The thing is.. he needs to talk with the right people...
Addicts will cut EVERYONE who MIGHT HELP them OUT of their lives when they start their addictions...
Addicts quickly become reclusive and this makes things worse... no one can throw you a rope if you go swimming alone so to speak.
The OW is likley not getting much support or help on her end either... She was likley lost and confused too... and the both of them got the mistaken idea that if they were together that would solve their problems... it won't... we know that.
The more support he gets, the less he will need her. If we can get pressure on HER to leave HIM alone from HER husband that would help too... my main concern is that her H may react by divorcing her outright... unlikley though.
Her H is giong to feel the same way YOU do... You didn't want to divorce your H, and he won't likley want to divorce his wife either... But some positive pressure on her on the home front would reduce the amount these two connect.
Make sure you don't appear NEEDY to your H though. It's ok to appear HURT, but dont give him the idea you will accept whatever terms he offers... If you appear hurt he's going to take advantage of that... keep HIM on the defensive about how YOU feel, what YOUR plans are, etc... the more defensive he is the more cooperative he's going to be...
If you show him a doormat he's doing to use it... so don't be a doormat basically. I am not saying you are right now... just warning you there's a tendency for people in your position to do that.... I did.. and I regret it because the affair in my home lasted a LOT longer because I showed weakness...
You show him weakness he will think he can keep enjoying a marriage and an affair... you show him confidence and boundaries he takes a step back and has to think long and hard.
The more yoru friend support him the more positive he will be.. he needs to be around SAFE people right now... if your friends an let him stay with them that would work great too.