This was from the SC Flashcards.
In spite of Charlie's protest, his friends-including his closest companion Rafael- insisted that Charlie consider outside aid to help Charlie deal with his eating disorder.
The exaplanation said that "The use 'his' in the phrase 'his friends' is okay, because only possessive pronouns can refer to nouns that are part of a possessive clause".
What does that mean?! Can someone provide another example?