Regardless of the legal side of things can she get herself out of her, especially being there with a child as it doesn't sound like a very safe place to be?
When you refer to police do you mean just the police officer(s) who attended? I don't know the legal position but could she have a word with her local community officer about it or someone more senior?
The trouble is, if he is causing problems now then it could escalate and much as it may not seem fair, she may just be best getting out of there as soon as possible. She could try and speak to someone at the Citizen's Advice Bureau about her legal position in respect of her contract, landlord's actions etc...
Is her deposit protected in a scheme and has she paid much rent in advance?