To view customer data that affects invoice follow-up in the app:

1. Open Customers and select the customer
2. Check the saved billing email, address, VAT number, and payment terms on the customer profile
3. Open the customer's invoices or payment history view to see statuses, due dates, and recorded payments
4. Update the customer record if the contact or billing details are outdated

This answer is limited to the customer data the app records and where to find it.