well let me start out by saying your friend is an absolute dumbfuck...

that weed when burnt doesn't give off unhealthy smoke, so when you rip that bong you aren't doing any damage at all, and the best part of it is that it's natural, natural things can't be damaging

weed smoke contains tar,carbon and many other undesirable elements...but studies have shown that adults who have used weed regularly over the course of 8 years showed basically no signs of a decrease in lung function... so while smoking weed may cause a build up of tar in your lungs, it won't permanently damage them, or cause cancer...natural things can't be damaging??? tell your friend to try smoking some poison ivy, see if he's right that natural things can't be damaging...

cannabis can not cause cancer, and studies have shown that that cannabinoids can kill cancer cells, but equating that scientific fact with " weed cures cancer" is completely idiotic...

weed does not effect serotonin levels

so overall, you're friend is a complete fuckwit, and either finds joy in talking out of his ass, or is completely ignorant to facts...