If she’s such a good friend, she sure doesn’t know much about the gal or she’s not telling.
Drugs would have explained it, because a lot of these meth or crack addicts go on several day binges, and all they want to do is hole up in a room somewhere and smoke their pipes.
Since it's been so long, (over a week) that likely isn't the case.
I fear the girl has become the victim of foul play, and I think her communications may shed some light on the matter.
But getting into a car with someone at 3:00 in the morning implies they knew and trusted the person pretty well.