4.2ghz is a very slight overclock and thus should not need much of a cooler. Definetly not the NH-D14 which, in honesty, is a probably more than you would ever need. It sounds pretty clearly like your friend doesn't know much about computers, so you should probably verify that you know how to overclock anyway...
Although I personally think its a little ridiculous to spend $600 on a cpu, I would suggest something along the lines of the hyper 212+ or the Xigmatek dark knight. If you really wanted to push it or just felt like taking a bigger chunk out of your friends absurd budget, I would suggest looking at Corsair's Hydro series of coolers, and Antec's Kuhler series. Both are AIO liquid cooling systems. I know that you said you are afraid that they might leak or something, but after the 100s of 1000s of units they've shipped, I haven't heard of a single recent case of them leaking. I know that there might have been a few with the very early h50's, but I haven't heard of any h70s, or h100s that have gone bad.
Would you mind posting the rest of the build? I'm curious to know what you put together with $2600.