bad customer service

This is a terrible experience for my mom and me. Bad customer service. just happened 2 hour ago. Today is Tuesday. 10% discount for those who is over 55 years old. My mom is so excited to go shopping. I dropped her off there. Two hours later, I went to pick her up and found she was waiting at a register. The cashier refused to gave her the discount. (My mom doesn't speak English) I knew the issue and asked for the discount. The cashier told me that it was the second purchase which not qualify for discount. The fact is that it is my mom's first and only one purchase today. I explained it to the cashier, but cashier was very rude and said " I know it is the second purchase. you don't need to say it. I know. we can check the camera". Okay, check the camera please. 15 minutes later, someone told me they don't know, because the camera is down. Then no one server me and no one apology for the inconvenience. I told the manager, at least, someone need to say sorry. The manager said" you need a sorry? I can say sorry." in a very rude way. That is the only "sorry " I got.
To make clear
1. That is my mom's first and only purchase today.
2. Cashier should apology. She asked to check the camera and nothing showed.
3. manager's bad manner.-- Is this the way you say sorry:" you need a sorry? I can say sorry."

That is not for the money- $3. I complain for the bad service. I just need a smile and an apology for the rude manner.