You absorb the THC and other cannabinoids in smoked cannabis through a transaction in the tiny air sacks of your lungs, which is the furthest point the smoke travels to in your respiratory system. All your pal could remotely be spitting out would be traces of resin or particulate matter on his tongue. It might contain microscopic amounts of THC if you analyzed it in a lab, but he's not really wasting anything good or lessening an opportunity for a better high by spitting.