I�m not an expert on the legalities of renting property, however flyciderman�s argument allowing your friend to claim/pressurise her landlady into returning her deposit, seems to depend on the landlady having not met her legal obligations. If the landlady has complied, then your friends claim may be weak.
Let�s say that the landlady has failed to register the deposit, has no valid gas safety certificate, and for good measure, there is no smoke detector in the property � to whom should she complain? (I don�t know).
For sure, if she went to the local police station, they would be too busy chasing motorists for minor traffic violations to bother with landlords who put the lives of tenants at risk.