Tenant doesn't use heating - now have mould issue

Other

My daughter has let her flat for the first time. She has lived there since 2005 very comfortably. Tenants don't like to use heating as they say it is too expensive, consequently there is now a big issue with mould in the kitchen and bathroom which has never been there before. She wants to install a dehumidifier of the desiccant type because it is so cold but if the tenant refuses to use it because of the cost of running it where does she stand. The flat is being damaged and will only get worse if they don't help the situation. Other than this they are excellent tenants.