extendo friend
An individual who you only socially interact with when a mutual friend of both of you is present. Without the link holding you two, you both have no reason to talk making it awkward with out the friend.
"Hey! Have you heard from Nina since Matt's birthday party?"
"Nah man, she's an extendo friend to me. I don't really know her."
"Nah man, she's an extendo friend to me. I don't really know her."