add_filter(
   'notify_user_about_friend_request',
    function(
        $notify_user,
        $user,
        $friend_user
    ) {
        // Your code here.
        return $notify_user;
    },
    10,
    3
);

apply_filters( 'notify_user_about_friend_request', $notify_user, $user, $friend_user )