Cracked Glass on Built in Hob. Tenant or Landlord’s Responsibility?

Hello,
One tenant has reported her built in hob has a crack running the length of it and will need replacing.
The landlord is straight on to me, asking who is liable as it wasn’t cracked when the tenant moved in 4 years ago. Normally, as it’s a built in appliance, the landlord would be responsible but I’m not sure on this one, it could be accidental damage by the tenant.

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