BuddyPress.org

Please clarify, as I’m getting this from your post above:
That you have one WPMU installation. You installed BP in e.g. blog_id_2 and then removed BP there and now pointed it to another user’s blog in e.g. blog_id_4.
– Did you add/change wp-config.php to point to new blog_id_4?
define ( ‘BP_ROOT_BLOG’, $blog_id );
so e.g. define ( ‘BP_ROOT_BLOG’, 4 );
– Check Settings > Reading – have you changed the static front page from Activity to WP default settings?