Quote Originally Posted by Mr Smith View Post
Ева И Вадим шли по Тверской улице и говорили о её друзьях.

Should её not in fact be ней, the prepositional?

Ева И Вадим шли по Тверской улице и говорили о ней друзьях.
No, the original version is correct, yours is impossible.

Explanation:
"говорили о ней" - "were talking about her".
Compare:
"говорили обо мне" - "were talking about me" ("обо" is always used before "мне")
"говорили о тебе" - "were talking about you"
"говорили о нас" - "were talking about us"
"говорили о вас" - "were talking about you"
"говорили о нём" - "were talking about him"
"говорили о них" - "were talking about them"

Here we have personal pronouns in prepositional case. 3-rd person Personal pronouns take initial "н-" when they follow a preposition (о нём, о ней, о них).

"говорили о её друзьях" - "were talking about her friends".
Compare:
"говорили о моих друзьях" - "were talking about my friends"
"говорили о твоих друзьях" - "were talking about your friends"
"говорили о наших друзьях" - "were talking about our friends"
"говорили о ваших друзьях" - "were talking about your friends"
"говорили о его друзьях" - "were talking about his friends"
"говорили об их друзьях" - "were talking about their friends" ("об" is used because it precedes a vowel sound)

Here we have possessive pronouns + noun in prepositional case. 3-rd person possessive pronouns never take initial "н-" (о его друзьях, о её друзьях, об их друзьях).

Compare: in English you also have different pronouns, personal and possessive (me/my, you/your, us/our, him/his, them/their), but "her/her" is an occasional coincidence, and that is the cause of your confusion. You would not say in English "about him friends", would you? "о ней друзьях" is impossible due to the same reason.

BTW, "его, её, их" as possessive pronouns are undeclinable in Russian, only the noun changes.