Well my initial thought is to wait until your mom is cool and mention to her that the lady at the bank looked really hurt when you left, nothing else, not accusations, just state it and drop it. In my experience some people feel that getting their anger out and on everyone else is a good idea and if you go much further, you are likely just to get her Anger on you. Better to let her conscience work on her a bit.
If it was me, I'd buy some flowers and go back to the bank and talk to the lady, give her the flowers, let her know you care and that you are sorry about what happened. She is likely to tear up and need a hug, Customer service can be a really sucky job. But you have an opportunity to make this ladies life much better and maybe give her a story of kindness to tell for the rest of her life.
It upsets me that the "Ugly consumer" is babied by most companies. My thought is that if you permit it, they only get worse. We should be rewarding polite people, not rude and demanding folks.