Tell your friend that it's in the direction that works. Follow the logic below, and pass it along so your friend understands the concept.

>>>>Turning the barrel in the direction that screws it outward, (counter-clockwise, looking from the top) lengthens the housing, which has the same effect as shortening the wire, and moves the derailleur inward.

>>>>Turning it so it screws inward, does the opposite and moves the RD outward.

Adjust the RD trim in the direction that solves your problem, ie. inward for crisper shifting to larger sprockets, outward for crisper upshifting to smaller ones, looking for the most neutral balance between the two.

Assuming all was correct, and you suspect cable "stretch" that would have moved the trim outward (the same way as shortening the housing) so bring the adjuster out to correct.

If your friend simply remembers that the adjuster changes the housing length, which is exactly opposite to changing wire length he'll never go wrong.