The fix for this is to change the select statement on lines 81 - 86 in the file:
$gv_query_raw = "select c.coupon_amount, c.coupon_code, c.coupon_id, et.sent_firstname, et.sent_lastname, et.customer_id_sent, et.emailed_to, et.date_sent, crt.redeem_date, c.coupon_id
from " . TABLE_COUPONS . " c
left join " . TABLE_COUPON_REDEEM_TRACK . " crt
on c.coupon_id= crt.coupon_id, " . TABLE_COUPON_EMAIL_TRACK . " et
where c.coupon_id = et.coupon_id " . "
and c.coupon_type = 'G'
order by date_sent desc";