No. It's not possible to OD on marijuana.
It could have been laced with something to incapacitate her. I've seen it laced with 'shrooms and people hallucinating. Anyone know if the date rape drug is effective smoked? For that matter, slipping something into a water bottle or beverage would be easy enough to incapacitate.
"Want to come in for a drink?"
"Thanks for the ride, can I offer you a glass of wine?"
Doesn't really jive if she had set plans, if I had to be somewhere and left early to do it, having a glass of wine or such wouldn't play into it.