Unfortunately, we have no clue of how you could restrict comments to friends. 🤷🏻♀️
As this issue is related with BuddyPress, it’s best if you try to reach the plugin authors. They will be more informed and better suited to help you.
To go further, we invite you to post your request on their support forums: buddypress.org/support
Don’t hesitate to share the solution with us. I’m sure it will help other users facing the same issue. By doing this, you will be making a great contribution to this forum, and we thank you very much for that! #SharingIsCaring 💪🏻