If the passenger is just as drunk as the driver then they also have impaired judgment and I think could not be help responsible.

However someone sober who knows the driver is drunk should really stop them, or for their own safety not get in the car at all.

But I can see extenuating circumstances, the partner of an aggressive drunk who knows refusal means trouble for example. In a case like that, then no, the passenger should be cleared.

The biggest problem here is proving someone knew the driver was drunk, and intentionally let that driver drive drunk. That I think would be impossible to prove beyond reasonable doubt!