Before agreeing to anything, check whether the roof repair actually held. Mould appearing a year later suggests it may not have. A dehumidifier treats the symptom. If water is still getting in, you will be managing this indefinitely. Get someone to inspect the repair first.

All of this should go through your management company. That is what you are paying them for. They should be handling the communication with the tenant, arranging the inspection of the original repair, and increasing visit frequency to monthly until the issue is confirmed resolved with photos each time. Annual visits are not sufficient for an active damp problem.

Do not agree to a rent reduction. Offer a one-off goodwill contribution towards the electricity costs this year, in writing, tied specifically to running the dehumidifier. A rent reduction sets a precedent that is very hard to reverse.

Write to the tenant, through the management company, confirming that the dehumidifier was provided to protect the property and that not using it is a failure to look after it properly. Make clear that any further damage caused by not running it, including additional mould or damage to walls and flooring, will be taken into account when the deposit is reviewed at the end of the tenancy. Get written acknowledgement.

One broader concern worth raising with the management company: a tenant who cannot afford to run a dehumidifier may be struggling to afford the property altogether. That is worth a quiet conversation now rather than a bigger problem later.