on-forum-post-status-transition.php

<?php
function on_forum_post_status_transition( $new_status, $old_status, $post ) {
	// If we are not looking at a reply or a topic, bail!
	if ( ! bbp_is_topic( $post ) && ! bbp_is_reply( $post ) ) {
		return;
	}

	$forum_id = bbp_get_forum_id( $post );
	$author_id = bbp_is_topic( $post )
		? bbp_get_topic_author_id( $post )
		: bbp_get_reply_author_id( $post );

	# You now have the (topic or reply) author ID
	# and the forum ID and can do further work
}

add_action( 'transition_post_status', 'on_forum_post_status_transition', 10, 3 );