add_filter(
   'notify_user_about_friend_message',
    function(
        bool $true,
        $user,
        $friend_user = null
    ) {
        // Your code here.
        return $true;
    },
    10,
    2
);

apply_filters( 'notify_user_about_friend_message', true, $user, $friend_user )

apply_filters( 'notify_user_about_friend_message', true, $user )