The passenger could of course *choose* to not speak to you--anyone can choose to not speak to another person. Or if he or she is contemplating suing you, his or her attorney would advise him or her to not speak to you--he or should could disregard that advice, but would be unlikely to. The only reasonably likely reason the passenger could *not* legally speak to you, however, is if there was a police investigation (e.g. for DUI) and the authorities ordered him or her to not speak to you. They  may have done that because they do not want the two witnesses to the DUI (you write that the other driver was drunk; so you and the passenger are witnesses to that) to have a chance to coordinate or compare stories, or to even inadvertantly "contaminate" each other's testimony: they would want each of you to have a fully independent recollection.