BMW, there are so many dimensions to her story and to yours. My dad had altzheimers and he became someone we didn't know. My mother, who adored him became afraid of him...you know from your friendship with Nikki what a very long goodbye this can be.
You
listened to your friend, you loved her, love her still. She wanted to spare her loved ones watching the inevitable...you respected her choice and I, personally, think you did the right thing in supporting her.
I am just so sorry that you are feeling such guilt and pain. She made a choice and so did you...unfortunately you have to live with the consequences of your choice. Life isn't fair but imho, she was blessed to have you for a friend.