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I would have assumed a heavier buffer with a smaller hole would trap more gas, allowing the bolt to rack with more force?
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Just the opposite. A smaller gas port hole will allow less gas through and therefore there will be less gas pressure to work the action. And a heavier buffer will slow down the bolt movement and soften the recoil impulse; if it's too heavy it will prevent proper extraction and ejection.
It A 7-inch 5.56 AR-15 has terrible ballistics compared to a longer barrel and is generally a pain to get working right; there is a lot less wiggle room in that small a package if everything isn't tuned just right.
It's hard to tell from your description of the problem if his AR is under-gassed or over-gassed. Does it ever properly eject? If it does, is that brass coming out relatively slowly and ejecting more than 45 degrees to the rear? If so, there's not enough gas pressure to reliably cycle the action. First, have him check the alignment and fit of the gas block. If he takes it off, he'll see a ring of carbon around the gas port in the barrel; if the gas port is completely inside that ring the gas block is aligned properly. Next, have him try a lighter buffer and/or spring; that's the most common fix for an under-gassed rifle.
However, ejection problems can also be due to the rifle being over-gassed. With too much gas pressure, the rifle will already be extracting the fired case while the pressure is high and the case will be ripped from the chamber while it's still expanded against the chamber walls. This can cause everything from a little flash out of the ejection port to chewed-up brass and failures to extract. When the rifle does properly function, it will throw brass violently straight sideways or even forward. The easiest fix for an over-gassed AR is a heavier buffer and sometimes a heavier spring too.