If an agreement has been entered into I am not sure what he thinks he is achieving by not letting her stay there in the daytime!
I am not sure why you would think this is not her normal home; unless there is some other place she lives in I would have thought it has to be if she is sleeping there.
I agree that there may be some doubt about her liability, but you have had a claim which you have a duty to determine, and as part of that you will be referring to the Rent Service, so the landlord will then have his answer one way or the other!
On reflection, and having read Peter’s comments, I am not sure there is really such a big issue around liability. I would suggest putting the landlord’s behaviour to one side and just determine the claim. Would also advise the tenant to get some advice from your private sector housing people about her situation.