Brother's house is an absolute dump, landlord won't fix - he's stopped paying rent.

Hi, the short version of this is that my brother's house is a massive mess, filled with black mold and all sorts of safety hazards that he's reported almost a year ago. The landlord (and management company) have done nothing about it, the problems have got worse, and it's reached breaking point. One issue he also has is a leaking tap, and when the quantity of leaking water is measured, it works out at over 18,000 litres of water since it was first reported, obviously affecting his bills.

He's finally reported it to the Environmental Health team at the council, but his tenancy is up in May anyway and he'll be looking for somewhere else to live by the time anything actually gets done about it now. So, he's stopped paying his rent.

I know that technically it's illegal for him to do so, and that theoretically the landlord could take him to court. My question is, if he was taken to court, how likely is it that a judge would recognise that the landlord has first broken the tenancy agreement (with all the standard clauses about the landlord providing a house in a fit state of repair) and ensure that full payment is not awarded? And, how likely could he take him to court anyway, if he simply doesn't give an address of where to reach him?