She is your best friend, not your daughter. She has a mother. Though her mother tragically passed away, this does not necessarily mean she would appreciate the gesture of someone else trying to fill that role in her life. My father passed away when I was a preteen, and even though I had some very good, strong male role models who stepped up and I loved and appreciated them immensely, that doesn't mean I would have been happy about the idea of them adopting me. On my birth certificate, my father's name is listed. His name will always be the one and only name I will allow to be on my birth certificate. Just because he is gone doesn't mean I am voiding his role and contribution in my life as my father. Your friend may very well feel the same way about her mother. Adopting her (if it's even possible) would mean her mother would no longer be named on her birth certificate. You would be venturing into very touchy territory by suggesting such an odd thing, and it may not go over well. It certainly wouldn't have gone over well with me. While I'm sure your heart is in the right place, you must be reasonable and realistic. Her mother is dead, but she is still her mother. You can still be family without making a legal declaration. If you want to do something touching to show her you think of her as family, why don't you write a personal declaration, telling her from this point forward, the two of you are sisters and family forever. You can both sign it and each keep a copy for sentimental keep sakes.