a lot of advice had been said on this thread already but not once have i read and taken into consideration that the guy may have children already with the girl, i mean they have been married for so long already that for now they must have kids going into school by this time. anyway, if this is the case then its what probably makes it harder for him to leave his wife.
i certainly agree that there is no guarantee for the girl not to ever cheat on him again, but then again is there such a thing as safe marriage or relationship? you may be anchored on the hardest of rock but there will always be storm and tempest that will shatter your foundation, and such is the nature of life and everything else that comes with it.
in the end it all just depends in you, whether you will decide to end your marriage or not, but consider your childrens if you have, have a serious talk with your wife, even see a marriage counselor if its possible. but worse comes to worst then cut the chord and move on, life has so much to offer and keep your heart and mind open.
read the book "The Road Less Travelled" by M. Scott Peck, it will help you alot