Billed for a stranger's coffee

We stopped by the wimpy at the 1 stop garage on N4 in Belfast for take away coffee. I asked my sister to buy me a cup of coffee from wimpy and to my surprise when she brought tback change and slip, the cashier billed for 2 because a stranger who happens to be a truck driver stood next to my sister and asked coffee from her. Your cashier billed the coffee on my account and then when my sister went back to inquire she was told that the cashier billed the truck driver's coffee on us and we need to wait for her to get the money from him. How dare your cashier bills coffee to my account without my sisters consent? What a nerve? I'm disgusted by your service, not the first time I experience bad service. I want my money back<br> <br> Your cashiers are very slow and you can wait for coffee up to 20 minutes, really? You should be knowing your customer behavior by now, coffee is on demand in the morning get more people to attend to customers period