bp-custom.php

// bp-custom.php does not exist by default. If you don’t have a file located at /wp-content/plugins/bp-custom.php, 
// @see https://codex.buddypress.org/themes/bp-custom-php/
// @see https://codex.buddypress.org/developer/loops-reference/the-activity-stream-loop/
// @see https://codex.buddypress.org/developer/function-examples/bp_ajax_querystring/

// Show activity to friends only :)  
function bp_loop_querystring_for_friend_only_activity( $query_string, $object ) {
    if ( ! empty( $query_string ) ) {
        $query_string .= '&';
    }

    if ( 'activity' != $object ) // if not in activity page, then return!!
    	return;

    	// Get your friends
	$friends = friends_get_friend_user_ids( bp_loggedin_user_id() );
	$friends[] = bp_loggedin_user_id();
	$friends_and_me = implode( ',', (array) $friends );
	$friends_and_me =  '&user_id=' . $friends_and_me;
 	$query_string .= $friends_and_me;
    	$query_string .= '&per_page=2'; // You can set pagaination. If not needed, you can remove it.
 
    	return $query_string;
}



// Let's start !
function bp_friend_only_activity() {
	add_action( 'bp_legacy_theme_ajax_querystring', 'bp_loop_querystring_for_friend_only_activity', 20, 2 );
}
// Not too early and not too late ! 9 seems ok ;)
add_action( 'bp_include', 'bp_friend_only_activity', 9 );