You might be misunderstanding just how limited the mes rea requirement is. All that's necessary is intent to engage in the criminal act, not the intent to knowingly commit a crime. That's what the jury instructions will say, too.

So unless she can argue that she somehow didn't mean to cut off the foot and it just happened by accident, mes rea is met here. Yes, it's a very low hurdle. People can commit crimes even if they don't believe what they're doing is a crime, and they can be punished for it.

And mens rea isn't even necessary in many situations where someone has a special duty of care for a specific person. Here, a medical professional may well have a sufficiently elevated duty of care for a patient that even a failure to act could give rise to criminal liability.