I drink about 40 ounces of coffee daily (consistently for the past 8 or so years), so I may not be the best guideline, but.. Since vaping and using no tobacco products for the past six months, my teeth stains have diminished. My oral hygiene habits have been unaltered in that time.
So could your buddy be experiencing dental browning from vaping? Yes, but he probably just should make sure he's brushing his teeth morning and night, and not vaping or otherwise using anything but water after brushing his teeth before bed.