Dean hadn't wanted to take Joan and Billy in the path of this hunt, though the ghost hadn't physically harmed any living person. There were just too many unknowns involved in the hunt for either of the Winchesters to want someone with limited experience involved. Unfortunately for that plan, her Boss had told her that she needed to be on the hunt with them, so they headed down the highway at the correct time and date with Joan and Billy in the backseat.
When it turned out that at least one of the ghosts involved in the reoccurring haunting didn't realize that they had died in the crash, making her more of an aware echo than a true haunting, Dean reluctantly allowed the spirit into the Impala next to the two of them. He really, really didn't want to do it. It went against every instinct as a Winchester to allow a spirit into close quarters with his nephew and Joan, let alone into the only home he truly recognized. But they'd already discovered that the ghost had been cremated, so without any easy method of forcing things she would need to be talked into crossing over. Dean had no problem confessing that Joan was hands down the best when it came to such a thing. Sam could be a close second, sometimes, but Dean himself had absolutely no patience with lingering spirits.
He listened with one ear, attention mostly given toward the other ghost that they were looking for, as Joan calmed down Molly by promising that she was safe with them and by showing her four-month-old Billy, who was fast asleep in his car seat. That kid could and did sleep through almost anything, as long as his mother stayed calm. Once Molly had relaxed Joan dropped the date in conversation.
It was probably Joan's matter-of-fact confirmation that the woman was, in fact, dead that helped the most. While they couldn't confirm what had happened without boots-on-the-ground research, from what Sam could tell the man that Molly had killed accidentally on the night of the car accident was angry and bound into chasing her every night on the anniversary. Take away Molly and they would have the time to come back at their leisure and find the man's remains for a salt and burn. Only, Molly hadn't realized that she was dead.
Joan managed to stay calm even when the angry spirit of the man came to the Impala, banging against the windows and door but unable to manifest inside for some reason. They'd never bothered to ghost-proof the Impala, which Dean was now cursing as an oversight that would not stand, but even though the female spirit was sitting next to Joan the male ghost couldn't get through. Whatever, he would worry about it later.
After about an hour of conversation Joan managed to talk Molly into crossing over and the male ghost went entirely quiet. Once the sun was up Sam and Dean moved through the woods while Joan and Billy took long-deserved naps and found the house and eventually the body of the dead man's wife, who had apparently committed suicide in her isolated home once she had buried her husband. Sam figured out that the man was buried in the front under the ten-year-old tree and they dug him up, put his wife in beside him, and salted and burned the remains.