I thought it was unlikely that, as a therapist who is used to drawing people out, she would not have engaged the driver in conversation to find out better whether he was someone she probably could trust. When he took a side road as a short cut, why didn't she quiz him about it at once instead of jumping out of the car ? He, of course, was naive and insensitive not to realize this would be frightening to his female passenger, and he should have explained what he was doing. Then he was tongue-tied when she ran away and just stood there. The problem is that he hadn't struck me as being so inarticulate in his scenes before .
On the other hand, to be absolutely safe, she should have just waited the 90 minutes for the tow truck instead of hitching a ride with a stranger. She could have waited behind the building instead of out in the open, and given exact information to her husband as to where she was on the answering machine instead of using up her coins calling him over and over.
Her behavior was not in keeping with what most intelligent people would have done, and it irked me.

Yeah; that was the bit when the whole film 'gelled' and broke my heart.

That's the correct answer, in my humble opinion. It's the sad part of the movie. If he'd gone to pick her up, they'd still be in their horrid marriage.

It could be said that it was the guy who murdered her daughter, by making her so fearful of strangers, although John Knox (Geoffrey Rush) might claim that his denial of her was her killer. Remember, he was home when she called him for help, but he didn't pick up the phone

NEITHER, I think.
I thought she just ACCIDENTALLY ran off that edge while running away from Nik in her frenzy.

What ajm83 said. And they did show this. You might not have realized what you were seeing (for one thing it was dark), but they showed Barbara running away from Nik in the brush, then falling suddenly off a cliff.
You must be the change you seek in the world. Gandhi

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Valerie fell off a cliff to her death, somewhere in the bush. She was fleeing from Nik, as she {mistakenly} feared for her life.
The real ending to me was the dance at the end awesome moment