As solidgold says: She has the right to commit suicide if that is what she wants.
However, she can do this on her own.
I am wondering if this is a way for her to take control over her own mortality, in a way that in fact in the end DOES NOT lead to her death by her own hand, so to speak. There is no reason why she should not find a way of taking her own life without implicating you, without going through this long process. I really dont think anybody has the right to ask that of somebody else.
Mostly, IME, people who really wants to commit suicide just do it. (I know, because I have had a few friends who did it and it was not really something any of them spoke about prior)
Maybe what she needs is just a way to talk about this, and coming to terms with it. Maybe she feals she needs to DO something, and the fact that she now is facing the prospect of just a few more months to live, is pushing her towards just DOING something.
I mean, how do you prepare for your last few months of life? It must be totally overpowering.