You are correct that antibiotics are for bacterial infections. COVID, being a virus, is not treated directly with antibiotics.

However, it is not uncommon to have a bacterial infection (could be anywhere in the body) at the same time as a viral infection - immune systems like to throw temper tantrums now and again in the lovely form of concomitant infections. I suspect that there is something else going on (or suspected to be going on) that you weren’t/aren’t aware of. It’s hard to say what exactly that may be without any other information about your mother, her medical history, and current condition.

If your mother doesn’t know, she can most certainly ask for clarification from the prescriber.