Using customer’s account for personal purchases?

I work at Costco and do IC on the side sometimes. Yesterday at Costco I was helping with the cashier I was assigned to. Lady comes up with her IC barcode, cashier scans it and starts scanning the order. The lady then uses her Apple Pay and tries paying with a card that clearly isn’t an IC card. Not only that but Costco needs the physical IC card for purchases since Apple Pay won’t work. The cashier asks her if she is using her personal card to pay for the order and the lady says yes, that she is going to use the customer’s account to make a purchase since she isn’t a Costco member. The cashier asked a manager if that’s allowed and the manager said no, saying only members can buy stuff at Costco. The IC shopper was going on about how she “knows” the customer and the customer doesn’t care if she buys stuff with her IC account. Again, we said it’s not allowed. Then she said that BJ’s and Sam’s lets her do it. Again, we say it’s not allowed. She made her real IC purchase and paid with the correct card, storming out, we heard her say “they better not have missed the water under the cart cause I don’t wanna get bitched out again,” even though we never bitched her out and couldn’t have been nicer.

Do IC shoppers normally make personal purchases at membership clubs with customers’ accounts? In my ten years at Costco I’ve never seen this.