Deleting a user in multisite

I ended up figuring this out a while ago… completely forgot to post it here.

Once I added the include properly it worked:

require_once('./wp-admin/includes/ms.php');
   if ( $user ) {
    wpmu_delete_user( $user->ID );
    }

For the sake of future readers here was the final code:

function handle_gateway_return() {

    $GWPass = get_option( $this->gateway . "_gateway_gwpass" );

    if ($_GET["GWPass"] != $GWPass) {     

    header("HTTP/1.0 401 Unauthorized");

    echo "<h1>Gateway 1.1</h1><h3>Authentication failed.</h3>";

    exit;  

    }


    if ($_GET["Action"] == "user.add") {    

    // Load variables.

    $ZFirstName        = trim($_GET['FIRSTNAME']);

    $ZLastName      = trim($_GET['LASTNAME']);

    $ZFullName      = $ZFirstName." ".$ZLastName;

    $ZUserName      = trim($_GET['username']);

    $ZEmail         = trim($_GET['EMAIL']);

    $ZPassword      = trim($_GET['password']); 

    $ZPassword      = md5($ZPassword); //md5

    if ( !username_exists( $ZUserName ) ) {
        wp_create_user( $ZUserName, $ZPassword, $ZEmail );
    }   

    // Tell server that user was added

    echo "OK|User added!";
     exit;
      } 

    else if ($_GET["Action"] == "user.delete") { 

        $ZUserName = trim($_GET['username']);

        $user = get_user_by( 'login', $ZUserName );

        require_once('./wp-admin/includes/ms.php');
    if ( $user ) {
    wpmu_delete_user( $user->ID );
    } else {
        echo "User Not Deleted!";
        exit;
    }

            // Tell server user deleted
    echo "OK|User deleted!";
    exit;
    }

    else { 

    echo "UNKNOW_ACTION|UNKNOW_ACTION";
    exit;
    }
  }