He can know a friend of a friend - He's a friend of a friend of his. I can know a friend of a friend - He's a friend of a friend of mine We can know a friend of a friend - He's a friend of a friend of ours.
We can also refer to friends of friends of ours.
Hi Chileno
Not quite sure what you are asking me, A friend of a friend would generally be someone you don't know well, more like an aquaintance or it may be somebody you don't know at all