You missed the point. I created that hypothetical situation to simply show the ambiguity of your argument. (And I responded to show the ambiguity of your 'fix everything' direction.) I'm not sure why you would comment it out, it's a story not a questionnaire..

Anyway, You can disagree if you like. But I think (you're allowed to speculate, but when you post it publically, you insinuate things/ unprofessional) your letting your personal friendship, feelings or whatever cloud your judgement. ("Cloud"in your opinion.) He's your friend and you feel the need to back him up...I get it. (Then you don't know me....if my 'friend' is wrong I won't argue against "truth" (I'm not defending HIM solely, it's anyone who deals with customers with your particular mindset to see in advance what they are taking on so in the future they can decide if it's worth their time.) what I feel is
Dwayne's a professional, he knows what he's capable of catching and what he's not capable of. He also knows the standard of workmanship and professionalism that he holds himself to. (This is classic patronizing...'you can do it')
I'm a professional as well, I do my best to hold myself to a high standard of workmanship and I'm the first to kick myself in the ass if I feel I'm not meeting those standards. (sincerely, that is respectable) I also expect the same high standards from those I engage to do work for me. (If his work doesn't meet your standards, I'd be happy to disappoint you)
Now if you think that I'm being too high in my expectations (no just TOO VAGUE), then I guess that's your problem because you don't have to carry the responsibilities that I do (lol really?) and you don't have to solve the problems that I have to deal with. (I guess not)
Walk a mile in my shoes, then you might be able to offer some worthwhile opinions. (I'm sorry the road is so rough for you)
The last thing I will say on the subject is this. You asked where the responsibility should end for inspecting a set of heads. This is basically what I do with brand new untested heads:

Visual inspection for flaws, shipping damage, etc.
Check finish of machined surface's and straightness.
Check all the machined holes and threaded holes.
Put on a head gasket, intake and exhaust gaskets to check chamber sealing and alignment of stud holes, ports etc.
Straight edge along the valves to check alignment and heights.
Drop in a rocker shaft to check fit, height of pedestal stands and alignment with valves. (how's he gonna do that with no valvetrain provided?) (besides ONE PART of "alignment" is 'geometry' and that is the BUILDERS responsibility
Check spring bases in head.
Guide clearance.
Seat inspection. (Now I'm not reciprocating being factious here, ALL what does this entail???...this can help ANYONE ELSE who is still reading.)
I may do other checks (? what are the other checks? ...list them, easy enough) if they have been CNC'd of are known to have common quality issues.(vague again...honestly I have NO IDEA the VERBAL? direction you gave him but it is just too vague to expect anyone to catch ALL the issues that could be with AGAIN a CHEAP COPYCAT of a produced head ESPECIALLY with vague direction)
If in doubt...call the customer....
You can get a pretty good idea if the heads are usable after this. You don't need to go crazy and put them through an MRI
It does not take long to do all this. It's not hard to do and I don't think it is beyond the ability of anyone who works on cylinder heads and has a high professional standard.
This is all done before any machine work is carried out, because if anything is wrong then the heads can be exchanged under warranty. Once they get cut you can't send them back.
It's also really important that this stuff is right if you are then going to send parts interstate or in this case overseas.
(OK now we are all learning something new...I recommend you add pressure test and dyno/integrity check before shipping an engine you build because that oversea's customer will be upset and hold you responsible for poor/thin castings. Even though this is not the nicest exchange between us, I feel there is something that could be learned here in practices and responsibilities