Adoption Question: A friend wants us to become her child's legal guardians should she die or become incapacitated. How can we do this legally?

Here’s the story, our son’s best friend is a 6yr old girl. She is raised by a much older single mom. The mom has asked us on several occasions if we would agree to raising her daughter as our own child should something ever happen to her.

The mom is concerned that something will happen to her prior to her daughter becoming an adult, due to her age (she’s in her 50s) and her health (she recovered from cancer just prior to getting pregnant with her very unexpected little girl).

The biological dad, grandma and an aunt are still alive…as are family members on the mom’s side. The mom has already informed her family she wants the girl to come to us, and wants no fighting about it. The dad is a severe alcoholic, unemployed for almost 3yrs, pays no child support, lives with his mom, and rarely sees his daughter.

She’d like to add this into her will, but I didn’t think that would make any difference in a court should something happen since the girl still has family members.