I would guess she was sliding rather than the brakes not working. If the brakes weren't working, she'd know before the wreck. And the odds of them "failing" somewhere between her last stoplight and the wreck is slim to none.

If she hydroplaned, I bet she'd realize it, its kinda of a violent thing.

If it happened on a dry road and there was no squealing, etc, then I'd be worried about your brakes, but in the wet, you can slide pretty easy depending on the road surface, the oils coming out of the road, etc.

When you are in a wreck, sometimes it's hard to remember after the fact exactly how fast, how far, how long, etc.

When the car gets fixed, do a little testing in an open space and just make sure everything is fine.

What model and trim was car she was driving?

I know you are just guessing, but a 10 degree grade is pretty steep still, enough to make a noticable difference in braking distance.