As the owner of a 2009 WRX, I'll be first to say that the power is not lacking on these cars in stock form (265hp on the regular WRX, btw
. If I wanted to spend money somewhere, I'd bring the brakes and suspension up to par with the power output.
The biggest problem on them in stock form is the brakes. They are really terrible, but they are really easy to upgrade. If he has money burning a hole in his pocket, tell him to get a new set of decent pads (like a set of of Hawk HPS's), some braided stainless brake lines, and some good hi-temp fluid.
These cars can get you in trouble fast if you don't know how to drive. They understeer really bad in stock form. The rear of my hatch in stock form felt a little squishy in the corners, so I threw in a STI rear swaybar. It helped, but it still needs more dampening all around. Since its my daily driver and not a track car, I'm happy where it is.
Another upgrade I did is wider wheels and tires. If he has a 2011, it should have 17x8's. Those are more than enough for these cars, and that's what I have on mine. His should have a 235/45/17 tire, but you can go with a 245/40/17 on those wheels. I did on mine, and it made a big difference in handling from my old 225/45/17's that came stock on my 2009.
Have him do that stuff, and then see if he can handle an increase in power. Last thing I need is another noob to crash one of these and have my insurance go up even more. 
2009 WRX (daily driver) 1987 Shelby CSX and 1979 Trans Am (money pits) AWD, FWD, and RWD in my driveway!