This is just pure speculation, on my part, but it could have been a drug-deal set-up gone bad. I don't know what her boyfriend does, but with two girlfriends who use, he is most-likely a user or a dealer (or both).
What if she was set-up, by someone trying to get some dope, without any money? They might have been trying to use Shasta to get to the supplier or make her fork over cash for drugs. Or maybe she was selling/holding?
Her family probably expected her home not long after she left, as it was late to begin with, and with her past, they're probably extra watchful of her comings and goings.
I say all these, just by thought. And with experience as an addict.