[glasgow guy]

I would say keep out of it as it doesnt have anything to do with you - what people get up to is their own thing regardless of whatever it maybe - are you wanting to tell his bit on the side because you dont like him/cheaters ? - if so thats quite bitter, you may end up doing more damage in the long run by telling his g/f who may find his wife and tell her - then you may be talking marriage breakup/ courts / money etc and if there are kids involved then it could get nasty - all because you dont like this guy cos he's doing the dirty and that he talks crap ?? - if you could live with that then fair enough - I couldnt do it personally.
I would be inclined to keep my nose out of it as have my own problems to sort out and these things have a habit of coming to light on their own and for all you know his marriage maybe a sham and over and this is why he's seeing this girl - just becuase he go's home at night doesnt mean to say that everything is fine at home maternal wise.
I know he's cheating (alledgedly), I know its wrong (if his own marriage is fine) but if you dont know all the facts you cannot going telling his g/f this or his wife that.
I am presuming that there both adults and know the consequences of what they are doing so why not just sit back and wait for the fireworks to go off - then you can say ' I told you so'.