I don't see in the documentation what the option name is to get_user_option() for the coupon code... Also, which is the most recently used code? Will it be $array[0] ? What about no coupon... Null?

Example PHP Code to Demonstrate

$user_id      = get_current_user_id();
$user_coupons = get_user_option('s2member_coupon_codes', $user_id);
// ↑ Coupons the customer has used to complete checkout.
// This is an array of unique coupon codes, in chronological order.
// If the customer has never used a coupon code, this will be an empty array, or `FALSE`.

if(is_array($user_coupons) && !empty($user_coupons))
    $oldest_coupon_code = $user_coupons[0];
    $newest_coupon_code = $user_coupons[count($user_coupons) - 1];