Why more than one friend? The placement of the apostrophe in the answer, "friends' conversation," signals more than one friend. However, rafiki is specifically singular, and mazungumzo, although plural in form, means a conversation or a dialog. I would expect mazungumzo ya rafiki to be a friend's conversation, that is, a friend having a conversation (with somebody). Does it specifically mean two friends having a conversation together, thus "friends' conversation?"
I would like to add that I suspect that DL slipped up with their positioning of the apostrophe. More natural than to refer to a " friend's conversation", which is rather odd to do, is to speak of a "conversation of friends" which I answered with and which DL accepts. A conversation of friends = a friends' conversation, but is a less clumsy way of saying it.