The best friend you can be to her... is the one you are already.
When she feels weak, be there to lift her
When she feels scared, be her strength and take away her fears
Cry with her when she wants to cry
Laugh with her when she needs some good 'ole friendship medicine
When she needs quiet, be peaceful with her
When she is angry, listen to her and let her share
The best you can be to her, is be as you've been with her for 10 years - a friend, a confidante and mostly, a person who loves and respects her wishes and will hold her her hand 'til it's time for her to leave.
Live with her whilst she is living, laugh with her when she can laugh and love her with all your heart.
Then she'll have peace.
What will you do when she goes?... well Arella, she won't ever leave your heart, hold onto the memories and remember why you love her and remember she is, and will always be, your friend.
Praying, in my way, for you both