bp_get_the_thread_message_sender_id()

	function bp_get_the_thread_message_sender_id() {
		global $thread_template;

		$user_id = ! empty( $thread_template->message->sender_id )
			? $thread_template->message->sender_id
			: 0;

		/**
		 * Filters the ID for message sender within a single thread.
		 *
		 * @since BuddyPress 2.1.0
		 *
		 * @param int $user_id ID of the message sender.
		 */
		return (int) apply_filters( 'bp_get_the_thread_message_sender_id', (int) $user_id );
	}