This is the person responsible for most
urban legends. The guy who is usually the subject of these
urban legends is the very same person referred to in
a friend of a friend knew this guy.
The friend of a friend is usually an excuse when someone catches you rehashing some urban legend. For example:
"This guy woke up in a bathtub full of ice, with a sign saying that if we got up, he would die."
"Bullshit, that's just some stupid urban legend."
"Well, a friend of a friend told me that. And he said that he knew this guy who it happened to."