Code to enable having the main bbPress forums on a separate sub-site. Requires: WP Multisite, BuddyPress on main site and network-activated, bbPress on main site and sub-site

gistify497541.buddypress-separate-bbpress-forums-site.php

<?php
/**
 * Remove and add a custom function for bbPress' BuddyPress activity filter
 */
function custom_bbp_notifications_fix() {

	if ( !defined( 'BBPRESS_FORUMS_BLOG_ID' ) || !BBPRESS_FORUMS_BLOG_ID ) {
		return;
	}

	remove_filter( 'bp_notifications_get_notifications_for_user', 'bbp_format_buddypress_notifications', 10, 5 );
	add_filter( 'bp_notifications_get_notifications_for_user', 'custom_bbp_format_buddypress_notifications', 10, 5 );

	add_action( 'bbp_template_before_user_topics_created', 'custom_bbp_switch_to_forums' );
	add_action( 'bbp_template_before_user_replies', 'custom_bbp_switch_to_forums' );
	add_action( 'bbp_template_before_user_favorites', 'custom_bbp_switch_to_forums' );
	add_action( 'bbp_template_before_user_subscriptions', 'custom_bbp_switch_to_forums' );
	add_action( 'bbp_template_after_user_topics_created', 'custom_bbp_switch_from_forums' );
	add_action( 'bbp_template_after_user_replies', 'custom_bbp_switch_from_forums' );
	add_action( 'bbp_template_after_user_favorites', 'custom_bbp_switch_from_forums' );
	add_action( 'bbp_template_after_user_subscriptions', 'custom_bbp_switch_from_forums' );

}
add_action( 'init', 'custom_bbp_notifications_fix' );

/**
 * Format the BuddyBar/Toolbar notifications
 *
 * @since bbPress (r5155)
 *
 * @package bbPress
 *
 * @param string $action The kind of notification being rendered
 * @param int $item_id The primary item id
 * @param int $secondary_item_id The secondary item id
 * @param int $total_items The total number of messaging-related notifications waiting for the user
 * @param string $format 'string' for BuddyBar-compatible notifications; 'array' for WP Toolbar
 */
function custom_bbp_format_buddypress_notifications( $action, $item_id, $secondary_item_id, $total_items, $format = 'string' ) {

	if ( 'bbp_new_reply' == $action ) {
		custom_bbp_switch_to_forums();

		$action = bbp_format_buddypress_notifications( $action, $item_id, $secondary_item_id, $total_items, $format );

		custom_bbp_switch_from_forums();
	}

	return $action;

}

/**
 * @var boolean $custom_bbp_switch
 */
global $custom_bbp_switch;

/**
 * Switch to forums blog, if it's not the current blog
 */
function custom_bbp_switch_to_forums() {

	global $custom_bbp_switch;

	$custom_bbp_switch = ( get_current_blog_id() != BBPRESS_FORUMS_BLOG_ID );

	if ( $custom_bbp_switch ) {
		// Switch to forum site
		switch_to_blog( BBPRESS_FORUMS_BLOG_ID );
	}

}

/**
 * Switch back to current blog, if not the forums blog
 */
function custom_bbp_switch_from_forums() {

	global $custom_bbp_switch;

	if ( $custom_bbp_switch ) {
		restore_current_blog();
	}

	$custom_bbp_switch = false;

}