It's real easy to arrested for PI, because it is an opinion-based offense. Proving it in court is a whole different animal, and they get tossed more often than even doing a deferred probation. In situations I have handled, and there have been many, where a passenger gets arrested for PI it has been because the driver got arrested, and the police didn't want the passenger to drive away in the vehicle. I can't recall ever having a PI case that wasn't dismissed.
The drug that was found, if not on his person, but in the car in general, can also be defeated. Possession requires actual care, custody, and control. If not on your person there must be sufficient links to indicate care, custody, and control. For those who want a concise primmer on what it takes, I recommend reading a case called Allen v State. It is a Texas case, and it is the current law. Often police will arrest the nearest person to the drugs, but proximity just won't do it, especially if you are in someone else's car.
Right now, I'm withholding judgment. I've seen too many situations like this that have amounted to diddly. (regarding Boykin) The driver is likely looking at intox assault charges or agg assault with a deadly weapon charges.