Thanks for your question!
In this case, "può dirlo agli amici" and "può dirlo ai suoi amici" are both correct.
However, there is a difference:
"agli amici" means "to the friends"
"ai suoi amici" means "to her/his friends"
In this specific setting, it's implied that the friends we are referring to are her/his friends (maybe because the girl was surrounded by a bunch of friends), hence the speaker uses "agli amici".
In cases where this wasn't clear (whose friends are we talking about? My friends? Your friends? Their friends?) you would need to specify it.
Hope this helps :)