Do I need a bathroom extractor?

Hello, I have a flat in a listed building with an existing window in the bathroom which is suitably sized. IF it is opened as needed there would be no need for an extractor.

Two previous tenants lived here, a single guy and then a family of 4 with no issues of excessive condensation, ie mould on the walls. The current tenant with 2 small kids is not putting on heating in the flat nor ventilating the bathroom.

Just had an ECO 4 bod go over and he says they can top up the insulation in the loft space for free but the bathroom may need an extractor. This inevitably would need LB consent (I own the building so no freeholder consent is additionally required).

Is an extractor actually needed though?

New build regs seem to state if there is a window in a bathroom then no need for an extractor – surely the same for old buildings?

If I did install an extractor, the chances are she won’t use it (electric use = cost) so the whole things seems an exercise in futility.

Suggestions anyone?

Reluctant Landlord