I wanted to be able to use the email address for a user making a booking within MRBS to send acknowledgements, confirmations etc.

I added this code within function_mail.inc and it works. The bit I inserted into the existing function is marked with comments at the beginning and end.

// get the email address of a user
// returns an empty string in the event of an error
function get_email_address($user)
{
  global $mail_settings, $auth, $tbl_users, $ldap_get_user_email;
 
  if ('db' == $auth['type'])
  {
    $email = sql_query1("SELECT email
                         FROM $tbl_users
                         WHERE name='" . addslashes($user) . "'
                         LIMIT 1");
    if ($email == -1)
    {
      $email = "";
    }
  }
  else if (('ldap' == $auth['type']) && $ldap_get_user_email)
  {
    $email = authLdapGetEmail($user);
  }
  // Inserted code for Drupal module ////////////////////////////
 
   else if ('drupal' == $auth['type'])
  {
   //Connect to the Drupal database
   $conn = sql_connect($auth['drupal']['db_system'],
                      $auth['drupal']['db_host'],
                      $auth['drupal']['db_username'],
                      $auth['drupal']['db_password'],
                      $auth['drupal']['db_name']);
  $user = addslashes($user);
  
    //we use default D7 table names here to take email address of the user
  $drupal_email_query = "SELECT mail FROM dr_users WHERE name = '$user'";
  $r_email = sql_query($drupal_email_query, $conn);
  $row_email = sql_row($r_email, 0, $conn);
  $email = $row_email[0];   
 
  }
  // End of Inserted Code ////////////////////////////////////////////////////////////////////////////
  else
  {
    $email = str_replace($mail_settings['username_suffix'], '', $user);
    $email .= $mail_settings['domain'];
  }
  return $email;
}

I hope this can help other users.