I should imagine your Mum will have needed to enter her TV PIN in order to do anything that would add charges to the VM bill.
In order to view Netflix programmes on her box, your Mum will need to have a valid Netflix subscription registered on her box - either one that she pays for, or if you're willing to test Netflix's new password-sharing rules, that of a willing friend or family member.
Ultimately Netflix is a chargeable service. Some of the newer high-end bundles do include Netflix in them, and if your Mum has a TiVo there's a good chance she may not have renegotiated her package recently. That may be another option, depending on her circumstances.
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