When I was a child we were told to "stay out of grown folks business". Assuming the son is still a child it's best to avoid initiating that kind of drama. Teenagers have no way of knowing what the potential consequences might be.
The son also has no idea what his mother already knows or if his parents have a "sexless marriage" and the wife has chosen to turn a blind eye.
The messenger of news such as this is seldom "rewarded". Eventually the truth comes out. I would also probably not confront my father about it as I would have to explain snooping in his email.
However if everyone involved is an (adult) then the son can (intelligently) weigh the potential consequences of informing his mother. You really have to have a strong idea of whether she will be able to deal with it. Will (she) be better off knowing or not? Would she appreciate the truth coming from (you)? You have to know.