I generally agree with everyone above, but with one exception. If he is being openly criticized because of his dress, and there is an obvious/easy fix, let him know.
I was a runner at a law firm, and wore what I considered at the time to be business casual (button down and dress pants). Since I had been accepted to a local law school, I was regularly invited to lunches with the summer clerks. I was the only one without a tie. After a couple of these lunches, one of the younger attorneys told me that I needed to throw on a tie to avoid looking out of place. I was not at all offended.
Notice that it was a very simple suggestion - "wear a tie" - not how to coordinate colors, or to use collar stays, or something that's asking the person to change a style. Simply a suggestion to step up the formality a small notch. V-neck undershirts as advice is just asking for trouble, IMO.