Hi John,
Thank you for the question.

Here is a sample code to do that.

add_filter( 'bp_activity_can_comment', function ( $can, $activity_type ) {

	// already protected, return.
	if ( ! $can ) {
		return $can;
	}

	$logged_user_id   = get_current_user_id();
	$activity_user_id = bp_get_activity_user_id();

	// if not user's own activity, or not super admin
	// and the current user is not friends with the activity user, disable comment.
	if ( $logged_user_id != $activity_user_id &&
	     ! is_super_admin() && function_exists( 'friends_check_friendship' ) &&
	     ! friends_check_friendship( $logged_user_id, $activity_user_id )
	) {
		$can = false;
	}

	return $can;
}, 20, 2 );

Please note that it will do it for all activities(including user’s activity on the group).
Also, It will not hide the comment reply button.

Regards
Brajesh

Hi John,
Thank you.

It’s a pleasure. Thank you for the kind words.

Regards
Brajesh