Well Raz, you know the guy better than any of us do...

Normally, I'd go for up-front honesty and be the one to tell him, that way, if she gets vengeful or something, it doesn't come around to bite you in the ass later. If she gets real upset about it, she could twist it around to say you started it. I'd try to avoid that, if possible, as people will usually believe what they want to believe, rather than think logically... Most guys would side with their wife, unless they're a super-skank.

If you think he'd blow up totally at his wife, I don't know what to do then.

It really is a judgement call.