My FI is taking my name when we get married, and his daughter (who has lived with us since she was 11 months old) has told us that when we get married, she definitely wants to have my surname too, since she currently has her dad's surname. I'm actually really happy about this, and so is FI, but we're worried about how to approach the subject with her mother. She's a really good friend of mine, actually, and I've known her since I was 13, and she only sees my step-daughter for 3 days a month (her choice), but I'm fairly sure she won't be very happy about it, and we need her permission to change SD's name.
What would be the best way to breach the subject with SD's mother?