You would have lost the betI would put money on him not checking carefully the left side of the P600 as he was instructed to. At this point in time with the P600 AND R3000 there is likely to have a pool of pigment ink that builds on the left side interior of the printer. As this pool sits there and dries up slowly, it slowly builds up and then causes perceived clogs but what happens is that the collision of the built up pile of pigment disturbs the nozzles on the printhead and the printing nozzle chamber empties and will not print. Essentially the top of the ink pool that is tacky will pull ink out of the nozzle of clog the nozzle as it touches the peak of the buildup.
One symptom of this is that the nozzle check pattern will have a horizontal band of missing steps aligned across multiple channels. Then one day the problem disappears..... and then returns. Why? When the moistened buildup dries up more it shrinks and its peak drops avoiding collision with the underside of the printhead. When some cleaning or printing is carried out and the printer drops more ink on the pile it builds up again as it is moistened and then collisions reoccur and it will cause bad printing.
This problem in known with experienced Epson users and unfortunately many don't know or perceive what has happened. When I used to purchase used Epson R1800. R2400s etc. pigment machines that is the first operation that needs to be carried out...to remove the pigment deposits both from normal printing and also borderless printing.
This happened to my R3000 recently. The pile of built up pigment needs to be redissolved and extricated. One can do so with simply Windex or Generic Glass cleaner and a roll of paper towels with a smooth rod to work or agitate the absorbent pad to loosen the pigment. If one has a vacuum pump available like a dentist station, one simply wets the mound of pigment and then vaccum the dissolved deposits away into a liquid trap. The latter is what I use.