Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
On this type of thing I enlarge the art and print out just a regular letter sized (or whatever you use there) portion of it just to show him the dot pattern.Then, it's up to him. He may surprise you and like the effect. At any rate, it won't be on you.
So,why take on the work?If you already know he is a PITA then politely decline todo the work.
How large is the final image going to be? If its currently 10" high at 300dpi, you can adjust it to 20" high at 150dpi, which is pretty big on a shirt. Although I'm not experienced with sublimation, if its anything like DTG, 150dpi - while not ideal - should produce a print that the vast majority of people would be happy with. Have you tried one yet?
you can do quite a bit in photoshop without losing too much detail, especially with the right plugins. You might have to dig around on a few photography forums, but there are tools out there that would make this no big deal. The industry standard upsampling plugin is called "perfect resize" and I think it is $200 or so, but there are plenty of others and some are free.
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