Student let

Tenancy Deposits

We let houses, not HMOs and I wondered if someone can help on an issue? My daughter has been in a shared house and the landlord is trying to withhold the deposit for one of the 10 tenants who has not paid his rent, 4 other tenants have lost their Keys (not her), and he said 24 hours of cleaning - I agree it would have needed cleaning and its fine for them to share the cost - but not 24 hours! He has said the carpets need cleaning but may have to be replaced - I dont think they are that bad. I suppose my questions are

1. Is the deposit she paid just part of a pot so he can use her deposit to contribute to others lost keys?
2. Does he not have to apply to the deposit scheme to have this approved
3. re the 24 hours cleaning ??? does he have to submit an invoice for this?
4. How do we challenge this - he was a rogue landlord, they had multiple issues with sewerage coming up the shower, walls leaking water through, none functioning showers/washers.

Any help before I email him would be much appreciated!