Question: This might be a dumb question; I actually don't own SC2, so what experience I have is playing friends' accounts. But my friend has been playing on Diamond + Platinum, and so far he seems to be unopposed by 4 warp gate rush. Which is basically 4 warp gates + proxy pylon within 6 minutes, and sending in 3 zealots 3 stalkers.

Anyone who tries to tech up simply gets destroyed, because he has no units by the time the warp gets there. Is it because he hasn't been encountering anyone who is actually good?

And what are good counters to this, if I ever end up buying SC2 and playing against him?

I feel like this can't actually be a competitive build, because it doesn't seem to be the win-every-time strategy in professional games.

edit: I guess the more specific question I am asking is, "What are essential/crucial weaknesses in this build? (in general) In particular, the weaknesses that prevent it from being the end-all build"

Answer: 
What people often forget about the 4 Warp Gate strategy is that it's an all-in strategy. If your rush fails, your opponent has a better economy than yours and will crush you in mid-game.

If a 4 Gate is scouted and reacted to in the appropriate way, it's 70/30 in favor of the defender, and professional players recognize a 4 Gate when they see one.
Defensive play is always more efficient than offensive play. The static defence advantages, especially of forcefield on the ramp in PvP, is extremely strong. 3gate for toss or equivalent for other races are good enough to hold 4gate when prepared correctly; if you know the 4gate is coming you can easily efficiently defend it and come out ahead.