add_filter(
   'friends_suggest_user_login',
    function(
        $user_login_for_url,
        string $url
    ) {
        // Your code here.
        return $user_login_for_url;
    },
    10,
    2
);

apply_filters( 'friends_suggest_user_login', User::get_user_login_for_url( $url ), $url )

apply_filters( 'friends_suggest_user_login', User::get_user_login_for_url( $friend_url ), $friend_url )

apply_filters( 'friends_suggest_user_login', User::get_user_login_for_url( $url ), $url )