yeah, it seems as if its an illegal sublet, but he's claiming he's added her to the official documents now, saying that makes her jointly liable.
It all sounds very odd. With
mieszkanie komunalne (rented from the city) he would need not only her written permission to add her, but also permission from the city housing department - something that's hard to get - just like becoming a council tenant in the UK, except the flats are in even shorter supply and the waiting list longer.
Housing associations rarely rent flats to people nowadays, in Warsaw at least - almost all occupants own the flats as a kind of leasehold and just pay service a charge and in any case, again she couldn't just be added without consent.
Unless....
...what he's in fact done is gone to the town hall and registered her as living at that address (if he has the right to do so), a document called a
meldunek, in which case there is no financial obligation whatsoever - it's rather like being on the electoral roll with a few differences that do not involve money.