A friend is driving and you are in the passenger seat of their car. May you drink a beer?

Why

The ban is on the act, not on the seat. Drinking any alcohol in a vehicle is illegal, and so is smoking or eating a cannabis product, for whoever is at the wheel and for anyone riding along. Being over 21 with a sober driver does not change that. Neither does where the container sits: the rule about keeping an open container in the trunk or a place where passengers do not sit is a separate requirement about carrying alcohol, and a container you are carrying must be sealed and unopened in the first place, with the glove box specifically ruled out for an open one. There is one written exception to the drinking ban and it depends on the kind of vehicle rather than the kind of drinker: it does not apply to a passenger in a bus, taxi, camper or motorhome.