My god, this is horrific.
Let's face it, some people do consent to some pretty vile and awful stuff all the time. It may not technically be consent in the eyes of the law, but nevertheless they do go into some pretty extreme and violent sexual experiences completely voluntarily and lucidly, with a total understanding of what it is they are doing.
But people who are vulnerable, mentally ill or emotionally damaged are probably far more likely to end up 'consenting' to violent and depraved stuff, and not quite so lucidly unfortunately, which is why the law has to be there to protect them and deter people who might take advantage of their vulnerability. Perhaps this young woman was someone who was being very cynically taken advantage of by a man who is abusive and predatory.
I imagine fisting is quite mild compared to the things some fucked up in the head people 'consent' to. However, it can be also be a normal part of the sexual repertoire for some perfectly well balanced, sane, non-screwed up people too. I don't get it myself, but that's not to say others don't enjoy it and indulge in it reasonably safely and carefully.
But the point is this: just how hard and how roughly do you have to fist someone to rupture their internal organs to the point where they are in the resuscitation unit FFS? I doubt she ever signed up for anything quite that rough, unless she was actually mentally ill. There's rough/experimental consensual sex and there's just losing control in a sadistic way and hurting someone with sexual violence that's really no different to violently attacking them in a non-sexual way. Just because it happened in bed doesn't excuse it as an accident.
It's like saying 'I meant to just slap my wife on the bum for a laugh, she likes it, honestly, but I got carried away and accidentally punched her 9 times and fractured her skull instead.' 
It sounds as though she consented to the fisting but that he was overcome with a scary loss of control and he didn't stop when she must have been showing very obvious signs of distress. Which is sick, unforgivable and horribly abusive.
But we can't possibly know, this is all just speculation.