Check the customer has Admin rights to access these details on their online account. You can see if the contact in question has Admin rights directly when searching for the customer via the portal’s staff mode (it will have a green tick-mark in the Admin column of the list that will show once you search for the customer).
If not, the current Admin will need to go to Team > select the person in question > and then modify their role to Administrator in Permissions and click Update.
Should the customer struggle with this, you can also help them with actions above in staff mode and if needed direct them to Account Helpdesk (making sure that screenshots of error messages are attached). In the meantime, make sure to assist with things you can assist with directly (e.g. you can provide copies of invoices, etc).