He discovers another woman can wear a cock and please his wife just as well...  If not better

He discovers watching his wife is very arousing...  And that another's tongue can do the same job

The much sought after threesome...  Perhaps a different variety, though.

He discovers tits rubbing on his back and moaning in his ears makes his cock hard

He discovers he can last longer and extend your pleasure with a strap on

Your friend can still remain modest and learn how to be dominant

Men are so much more desireable As the submission increases.  You are so much more helpful and attentive

As long as I hold the keys, I don't mind my friends teasing him.  Actually adds passion to our marriage