I have this friend who I have known for over 10 years, she has been a constant in my life and my daughter loves her dearly. She has been taking care of both of us for over a year now.
I'm dying and my daughter's father sexually abused her. I don't have any other family that can take her. My daughter is mentally disabled and I'm afraid of what would happen if she was taken into foster care.
My friend will take her when I pass and I know I can put it in my WILL but I want to be absolutely sure she has permanent custody before I die.

First, whatever you put into your will not be binding. A child is not property that can be willed. You can express your wishes, but they are not binding.

Your friend can apply for adoption, but the father will have to be contacted, Under the circumstances, I doubt if a court will accept his objection.

So you need to consult a Family Law attorney and have them start the ball rolling.

Yes, contact an attorney while you are well enough to sign papers and consent to what you want. It may be helpful for your friend to start on the steps to be approved as an adoptive parent, which would generally require a home study. She might also want to become approved as a foster parent. I would talk to an attorney to find out exactly how to proceed in your state but there may be things you can do now to make it easier later.

It will be important also that your family understand your wishes so they don't attempt to interfere with your friend adopting the child.