Chowking

My mother visited one mall chowking in valenzuela today 03-07, she will be buying 2 sets of 2 milktea for only 60. When she got into the front of the cashier, the cashier doesnt know about the coupons, she already tell my mother that there's no promo at all for the milktea, then my mother presented the coupons and the (female) manager got involved. Then the manager informed my mother that the coupons can be only be redeem once per person. However my mother and i got to one of your branches last 03-04, and we got to avail the 2 coupons in 1 transaction, so just to be clear here. Can the coupons be redeem once per person or once per transaction because it only indicates that the coupon can be only used once. And for the manager at one mall valenzuela branch she is threatening my mother about that situation and not explaining the mechanics to my mother. It is a very dissapointing way of giving customer service. And also she got the coupon and then punching 3 large size of milk tea wherein the order should be the promo of 2 milktea and a large milktea.