Air is delivered under pressure by mouth in scuba. Nose is not used even in a full face mask. Your nose is an inch long. All the co2 in it is expelled at speed and volume when you exhale normally. You also don't have a restriction like a tube condensing the gasses. Co2 is heavier than 02. It won't travel as far in a tube, a moist tube reducing the inner diameter even more from breath, and will be rebreathed. People over breathe snorkels all the time on the surface. People over breath regulators under water as well. I've done it. I've rescued people who've done it.
If he uses a really short tube down and out the neck just to inhale and then breathes out his nose, should be fine except vapour build up.
Wearing restrictive armour, in a hot day, will add to the work of breathing. Air in a tube has dead space to overcome ...basically air that doesn't contribute to gas exchange. People have tried to use really long snorkels...and passed out. Any more than about half a meter and it is really hard to suck good air down and you end up exhaling like mad to clear all that crappy air.
Scuba delivers gas at ambient pressure so there is no gradient to overcome. Re breathers recycle the gas with less than 21% oxygen on the surface and add o2 to bring it back up to safe breathing.
But, he's above water, and his tube will be a foot long hopefully not bend around any corners. A larger tube will work better than a narrow tube.
But no better than a fan.