The damage to his lungs is probably similar to what the damage would be to smoking cigarettes heavily. I don't know the science behind pot smoke vs tobacco smoke and their different specific health risks, but I do know that smoke is smoke. Every kind of smoke contains carcinogens.
He probably has no sort of physical stamina whatsoever, and his lungs are probably as black as a heavy cig smoker.
That is assuming he really smokes ounces daily, I don't know if he does or not.
So let's see..
One cigarette has a mass of about 0.9 grams.
0.9 g x 20 = 18 g for a pack of cigs
Let's say he smokes an oz a day. 1 ounce is equal to just over 28 grams. That means the volume of smoke going through his lungs is roughly equal to the volume of 1.5 packs of cigs per day. That cannot be good in any way. That has to be pretty bad for one's health. Not that he shouldn't do it, I don't care. I wouldn't tell him not to. If it's what he wants to do, and has the money, whatever.
I think the reason weed is mostly viewed as less harmful, is because most daily tokers don't have the same volume of smoke going into their bodies as, say, someone with a 1-2 pack a day cig habit. Smoking a few joints every day/couple of days is surely not as bad as constantly chiefing cigs all day.
It's like alcohol. Alcohol is perfectly safe in moderation, but there are still alcoholics who take it way too far. Weed is probably generally safe in small amounts, but ounce(s) per day is a whole different ballgame.
Just for the record, I don't partake, but I do think marijuana should be legal. I'm not all gung ho for it, it's more that I just don't care, and don't see it as evil.