Most Helpful Guy
Really, i'd just walk. In this case, unless you know the other girl, you just need to take care of yourself. You don't know what he has said to the other girl and you don't know how she'll react. Who needs the drama that you might be inviting into your life.
Most Helpful Girl
Normally, I wouldn't say "An eye for an eye," but in this Particular case, where he has no tOnly been Cheating on you, he has Been... Cheating you both on his own end.
He has been caught red handed with his Underhanded hand in the crooked cookie jar and after you tell him "So long," he will walk out that door and be heard of No more, and probably do the same thing with another fling to this poor soul who never asked for Anything but a Good man.
Send her One message if it willl give you some closure. However, I am not so sure she will believe you. He sounds like a rat who will just smooth this over in a sweet cheesy way, so in essence, he would have won and got all the fun with laughing at you who would merely stand There... Stunned.
He is a loser and whoever gets this piece of morsle, will see through him and end up With... Having Got his number in the end, my friend.
Good luck. xx