BP_Messages_Thread::is_thread_recipient( int $thread_id, mix $user_id = null )

	public static function is_thread_recipient( $thread_id = 0, $user_id = null ) {
		if ( ! $user_id = $user_id ?: bp_loggedin_user_id() ) {
			return true;
		}

		$recipients = self::get_recipients_for_thread( $thread_id );
		$active_recipients = array_filter( $recipients, function ( $recipient ) {
			return ! $recipient->is_deleted;
		});

		return in_array( $user_id, wp_list_pluck( $active_recipients, 'user_id' ));
	}