Need to know how to access apple balance

In what way won't it let her use it? What is preventing her? If there is an error message, always include that when you ask a question here. It will save us both a lot of time and back and forth.

I am presuming she is part of Family Sharing. Do you have a payment method on your account that is still functioning? See: Add a payment method to your Apple Account - Apple Support

If there's a comment in red, it isn't working. In that case ask your bank what is wrong.

There may be limitation on what can be purchased with balance.

This document outlines what you can buy with your Apple Account [Apple ID] balance --> What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

Assuming Ask to Buy is active: If Ask to Buy isn't working - Apple Support

Then check with Apple Support: Official Apple Support