She may need to see a professional, but it is always worth reiterating that the stakes are very low. Like there is nothing that you can actually do wrong that isn't a five minute fix. It's fine to make mistakes.

“It’s just coffee” usually helped the shy green beans I trained

I was like this when I started. Not to the level of panic attacks at work but I was extremely introverted, quiet, and I have an anxiety disorder that used to cause panic attacks. I like to say, as silly as it may sound, “this is your domain - you’re in control here. If customers start to make you feel overwhelmed or anxious just remind yourself that you’re in control of the interaction, you can ask them to slow down, repeat themselves - whatever you need.”
And then I make sure to support them in this.

Also, when I went through training and struggled with greeting people and small talk, my trainer said to think of it like a game - try to greet them before they greet you. And that helped a little.

But really the thing that helped me most was getting comfortable in that environment. It might just take a little time for her to get comfortable in that store.
Working the hand off plane for awhile might help her too. Calling out food and drinks and thanking people is close to the interactions of working on a register and might help her feel more comfortable with being louder.