I guess it depends on the situation then Anewme. If his wife is suffering from neglect, and that is why she is looking elsewhere, then your approach is probably the correct one.
If his wife is certain of his undying love, then it is likely she is looking around because she doesn't respect him. Given that she is in her 20's and hasn't dated alot, she is probably naive enough to be impressed and turned on by some "bad-boy".
This is what I was going to say, in so many words. Although I'm not so sure I'd say she was being naive...just because she was married young, and didn't date a lot beforehand. It's still no excuse to behave like that. Sometimes we may feel it's fun to flirt with another guy/girl, and we may like to have our egos stroked by someone other than our mates.... HOWEVER, asking for someone's cell #, or to get together with someone, or "hook up" with someone, goes way beyond having the ego stroked, or just having innocent fun. Especially if the person is keeping it secret from the other mate.
Both parties are responsible for the healthy marital household, and must not forget to show love and affection, to each other, no matter how young they were when they married. They were old enough to understand the marriage vows they undertook at the time of their wedding. Sometimes we get into a slump, we know we love each other, but times get busy, and we forget to say it, or give each other that needed hug.
My mom always said, " Marriage isn't easy...it takes a lot of elbow grease and hard work, to make it work." Sadly, some don't work out.