The thing is, what a person says is expected to make sense in the context of a conversation. If you indeed mean "other people's friends", you don't spring a sentence like this on a listener without it being clear who "other people" are. If somebody did that to me (like you didHow would you interpret the following sentence? --
Ева и Вадим говорили об их друзьях.), I would assume they meant "своих". This is the case of common sense taking precedence over grammar.
-- Кто о чем вчера разговаривал?
-- Ева и Вадим говорили об их друзьях.
(их = своих, Ева и Вадим's, what else could it possibly be?)
-- Вчера зашёл разговор о Петровых.
-- О чём именно говорили?
-- Ева и Вадим говорили об их друзьях.
(genuine "их", Петровых)