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.
See this build a lot on gomtv by protoss, anyway to be able to counter you can;

Terran- Bunker up with marauders + a few marines use 1 base + 1 gass and use 3 rax then siege expand.

Zerg- use 2/3 spine crawlers and roaches they take a lot of hits and try to get burrow tech also spread creep to help surround + transfuse.

protoss- um 4 gate them back lol, saw TT1 do a 3 gate and held it off with mainly zealots and blocking the stalkers from running with probes. but otherwise use sentries to forcefield your ramp and tech to collosi I guess.

hope this helps main thing is don't fast expand and only move out when you macro up a bigger army cos for the 4 gate push they cut probes so if you hold your ahead in economy and are able to safely expand then win.