To go into a bit more detail, your customer should have access to a control panel to manage her domain at her current registrar.
If she does, then she should look for the method of unlocking the domain on her control panel and unlock it if possible.
Some registrars make unlocking difficult (or very difficult).
Which registrar is your new customer with? If she is finding it difficult to unlock her domain, we might be able to help you explain how to unlock the domain.
If she has her domain with a hosting service, we would need to know the name of the real domain registrar, not just the name of the hosting service.
Good luck!