Hm...without looking at more than what Daiki says in Volume 9, I can't see where it says anything about Emiko's best friend. If I'm not looking in the right spot, (In the middle of the book, a few pages before page 105), please direct me. :3
I think maybe you just misinterpreted what Daiki said; he's referring to Satoshi's mother, like, "your friend's mother".
~Zakura-chan