Was able to easily implement flood control by borrowing some snips from the tellafriend module. In this case I hard coded the flood limit. This could be easily improved by adding a form field.

In tellafriend_node.module:

Add after line 512:

flood_register_event('tellafriend_node');

Add after line 447:

    //Check that flood control has not been surpassed
    $my_threshold = 3;
    if (!flood_is_allowed('tellafriend_node', $my_threshold)) {
      form_set_error('', t("In order to prevent spammers from using this tell a friend function, you can't make more than %number mail requests per hour. Please try again later.", array('%number' => $my_threshold)));
      watchdog('mail', '%name has attempted to surpass the flood control for tellafriend.module', array('%name' => $user->name));
    }