You need to point him to some of the discussions about the 6.0 and 6.4 problems so that he can educate himself. Just telling him "stay away those are bad engines" he may not believe you.
I'm not a Ford guy but there are Ford forums out there with plenty of discussion about these engines and their issues. Head gaskets, clogged oil coolers, EGR...
There are indeed many 6.0's and 6.4's on the road without issues, but those aren't the ones for sale by and large. The truck is for sale because there is something wrong with it, 99.9 percent of the time. When a 6.0 or 6.4 is involved, it's more likely to be an impending engine disaster than not.