There's really not much sense in an argument that says you "lose" something by learning newer things -- provided, that is, you're really making an effort to wrap your head around the newer things that you're studying, and not just cramming selected phrases and reflexes into your head in order to finish some current task at hand.
If anything he's become a better C# programmer, not a worse one, precisely because of his "bad" experience, because he now sees that overuse of certain habits can lead to bizarre or otherwise undesirable results (just like with any language).
So really he should be thinking positively about the whole experience, rather than negatively.