Tenants wont ventilate property

Maintenance

I have new tenants been in for 6 weeks and complained about mould on some clothing in the wardrobe (Its a one bedroom ground floor flat on a busy road and no previous issues in the property). I went round and the heat and humidity was so bad I could barely breathe. All windows closed and curtains drawn despite it being daylight and a very hot day. I advised them to open windows to ventilate etc but they wont open them for various reasons. NRLA advised me first of all to get a specialist round to find the cause so Ive told the tenants someone will be going round and they are happy with that.
Then.... I get an message saying theyve got OCD and mental health problems preventing them from opening the windows and curtains.... fear of people outside the ground floor window, fear of burglars and their house cat will get out onto the busy road. Then they say it is an 'emotional support cat' and if I insist on them rehoming it I will leave them unsafe in the flat !
Not sure how to approach the situation without being accused of discriminating in some way but given these issues and the fact the flat is ground floor on a busy road, it is obviously not suitable for their needs.
Just wondering if anyone else has had similar problems and how they dealt with it please ? If they wont open the windows and have got mould in the summer I dread to think what will happen in the winter !