Of bloody course she didn't consent to being strangled to death. But she may have consented to rough play, which can easily get out of hand. Also, as Coronation Street is currently showing, severe damage can be done with little pressure on the throat, so it might not be as violent as you assume.
It's a very sad situation, but you cannot automatically assume it's murder, because our children are being influenced all over the place online with cutesy "choke me Daddy" memes, and books written about violent sex (50 Shades, anyone?), and are being made to think it's ok. Not only ok, but sensual, and loving! It's no wonder some of them choose to try it. And it's horrific when it goes wrong!