The Macho HR-02 is capable of overclocks much higher than 4 GHz. Check this review at xbitlabs, which shows overclocks much higher than 4 GHz for a CPU that is a worse overclocker than the 2500K. Or look take a look at this HardOCP review.
It is quite capable, but the fan will spin up notably when the overclocks are higher. Two slow fans are usually quieter than a single fast one, hence the recommendation of a dual fan tower for higher overclocks.
Of course, if he wants silence, he should control the fan speeds through the motherboard and limit the max speed: I assume he will be willing to do this -hey, this is spcr! No "normal" fan at its maximum speed is gonna be silent. As a point in case, all my case fans run at 600 rpms max speed, and the CPU fans max speed is exceptionally set to 800 rpms at full load (i.e., running stability testing programs): I will refuse to overclock to anything that can't run in complete silence. The Macho fans are PWM, so good for this.
The 14 cm fan included with the Macho, the TY-140
, is quite good with respect to noise. In fact, it was reported to be one of the best in last xbitlabs 14 cm fan roundup, see the conclusions here
. That's why I choose the Macho over the Mugen 3: given the same CPU temperature, the Macho will be a bit quieter. Just check the Mugen 3 review at xbitlabs and compare it with their Macho review: in this page of the Mugen 3 review
they say "Scythe Mugen 3 maintains steady position in the very center of the pack. Its closest rivals are the Thermalright HR-02 Macho (a little more efficient and much quieter) and Thermalright True Spirit (a little less efficient and noisy).".
That said, the Mugen 3 will serve you well too if you choose it
If you are willing to spend more money in cooling, then go for the Silver Arrow, or the Archon: if you choose the Archon, you might try buying an additional fan for it and lower the rpms to get even more silence at the same temperature, as suggested in a prior post.
Besides, if you really, really want to get the quietest fans , I would recommend you get 14 or 12 cm BeQuiet Silentwings USC, and a 3 pin to 4 pins adapter that allows you to feed a PWM signal to a 3 pins fan. I have four 14 cms USCs, and they are a dream come true with respect to noise. The PWM version is good too, but some samples have developed a little ticking, according to some friend of mine who runs the best "silent pc" site in my home country -though he is very, very picky. Unfortunately I don't know whether this is common nor whether it is fixed now.
Lastly, you will have to accept limiting your overclock if you want silence: all I could get out of my i7 2600k was 4.4 GHz, more MHz just meant I had to push the fans beyond the "no noise" threshold. Your mileage might vary.