Sometimes the correct translation for "mazungumzo" is shown as conversations and sometimes it is shown as conversation. I don't know which is right, but it should be consistent-- or if both are possible, both should be accepted.
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?"
rafiki is not specifically singular (marafiki is a possible plural but just rafiki is more common), and I guess the context makes plural the natural assumption, but I agree that "(a/the) friend's conversation" should also be accepted.
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.
It could be just a typo (or a sign of confusion about punctuation), but on reflection, it's perfectly acceptable in a sentence like, "She interrupted her friends' conversation" (i.e. they were having a conversation and she butted in).
I agree. There is too little context to determine if it refers to more than one friend. "friend's" should be accepted
I would argue that it is highly unusual for one friend to have a conversation. The friend could be conversing with an enemy, I suppose, but we just don't get the context to say that it is a natural or correct translation.