I'd get to know him a bit better first, make him more of a mate than a neighbour and then it may be easier to say something. Take him to the pub once a week for a pint to develop that friendship.
You could always go along the lines of Environmental Health for it being a noise nuisance. I'm not sure how that works though, they'd probably encourage you to resolve the matter yourself first before they investigate. Plus the woman would know that it was one of the neighbours that has complained, although it allows someone else to break the news to the bloke in stead of you!
Just think whether you could live with the consequences....if you tell them and they split up and the wife ends up keeping the house things would be very awkward
. If the bloke keeps the house though you may have found a new best mate
Do you hear the husband and wife at it too or just the wife and the bit on the side?