"говорили о его друзьях" - "were talking about his friends"
"говорили об их друзьях" - "were talking about their friends"
One addition to this paradigm: the reflexive свой, "one's own" -- which can variously be translated as "his, hers, theirs, mine, ours, yours" depending on the subject of the verb:

1) Ева говорила о своих друзьях. = Yeva was talking about her (own) friends.
2) Ева говорила о её друзьях. = Yeva was talking about some other girl's friends.
3) Вадим говорил о своих друзьях. = Vadim was talking about his (own) friends.
4) Вадим говорил о его друзьях. = Vadim was talking about some other guy's friends.
5) Ева и Вадим говорили о своих друзьях. = Yeva and Vadim were talking about their (own, mutual) friends.

6) Ева и Вадим говорили о её друзьях. = Yeva and Vadim were talking about Yeva's friends.
7) Ева и Вадим говорили о его друзьях. = Yeva and Vadim were talking about Vadim's friends.

Note sentence 5 in particular: when you have more than one subject, свой refers back to all of the subjects, not just one of them. So you can't use a form of свой if you mean Yeva's friends only, and not Vadim's.
Sentence 6 would imply that they were talking about Olga's or Tanya's or Maria's friends, not Yeva's, so to avoid ambiguity you could say о друзьях Ольги, "about the friends of Olga." (And similarly for sentence 7.)

Finally, a question for native speakers. How would you interpret the following sentence? --

Ева и Вадим говорили об их друзьях.

I was always taught that this could only mean "They were talking about other people's friends" (i.e., individuals who are not friends of either Yeva or Vadim) and cannot be equivalent to Ева и Вадим говорили о своих друзьях. ("They were talking about their own mutual friends.")