Is he sniffing it or snorting it? Those are 2 entirely different things. I have heard of sniffing the fumes comet makes when it chemically reacts with other household substances. This can be very dangerous and damage your lungs.
Snorting the comet like you snort a line of cocaine I have not heard of and it sounds crazy. No more crazy than alcoholics drinking mouthwash or furniture polish though, and I know both of those go on.
I hope he gets the help he needs.