Having read your post today, and your first one, I feel very bad for you and your mom. For her, it is a death worse than 'regular' death, and she is dreading the same path that took her parent. That is understandable. This disease changes everything.
Normally, one would not even consider 'helping' someone take their own life. Of course not! You would try to get them psychiatric and/or medical help, or even make sure they were under supervision until the suicidal thoughts were gone. But this is no longer 'normal'.
Don't get me wrong. I'm not saying that you should 'help' her end it all. What I am trying to say is, it could be that you will be doing her a better service by honoring her wishes. If she is determined to take her own life, you won't be able to stop her anyway....she'll find a way (unless you have her locked up and put under constant surveillance), which wouldn't be doing her any favors, in my opinion.