A good friend

Well, are the trucks he's potentially buying already modified with the reliability items common for them? For the 6.4, if something goes wrong, it's really expensive. They only had that engine in fords for 3 model years, but even 2010 was cut short so they could quickly roll out the 2011 6.7 motor. So if anything goes wrong on a 6.4, the parts are nowhere near as common as 6.0 parts and its aftermarket. Also, you could point out just how large the "Reliability" aftermarket is for both of those engines. Those parts exist because there is quite a market for them. You don't really hear much about a "Bullet Proofing" kit for the Duramaxes, Cummins, or 7.3 / 6.7 Powerstrokes. Really, the only "Reliability" mod that people do with newer trucks is delete the emissions to remove the whole EGR and Aftertreatment systems. But those systems also aren't all that problematic except for people with a lousy duty cycle (Lots of idling / slow traffic). And for the earlier ford 6.7 with the single-sequential turbo, a few people started swapping out the newer turbo which is a typical VG turbo and doesn't have the bearing issues of the 1st gen 6.7 truck.

Secondly, as other have stated, does he or she plan to hold onto it for a long time? If so, your odds of failure do increase and if the vehicles are not bullet-proofed, you could potentially see a very expensive bill down the line. It's these fears that make these vehicles a "Bargain" in the used vehicle market. If set on Ford, refer him to get a 7.3 or 2013+ 6.7 diesel.