Ok I think I understand how to put what I am thinking down onto paper now...
For example your bench is lagging... You need to do triceps work. It's not that you need stronger triceps, strengthening your triceps will not improve your bench. Strengthening your bench improves your bench. If strengthening your triceps worked then you could just do singles with 10% of your bench each day to keep the groove and pushdown your way to an epic bench. What your assistance needs to do is hypertrophy the muscles involved to raise your strength ceiling so that your BENCH PRESS strength increases (not triceps strength).
The close grip work doesn't make your TRICEPS stronger so much as making your BENCH stronger. It increases the range of motion and changes the leverage of the exercise. Perhaps someone who is better at explaining things can have a go lol.
So when I say that his lats are fine and he should focus on his triceps, it's not his "weakness" as triceps strength has nothing to do with it. However hypertrophy of the muscle will increase the strength ceiling and allow your bench press strength to increase
I hope I explained this well. inb4 someone misses the boat entirely
"If you can 3-board press 10 more pounds than you can bench press, then you don’t have a “lock out” problem. You just missed a certain weight because you weren’t strong enough to generate the power needed to move that particular weight through the transition point in the movement."