My Sister has passed away suddenly Yesterday, we are waiting

My Sister has passed away suddenly Yesterday, we are waiting to hear from the coroner as to the cause of death, she was 57 and died suddenly in the bath.
Without wishing to sound like a vulture, I am a little concerned about her male friend that lives in her house, I think he was just a friend and there was no relationship as such.
He has stated to me today that he 'will' stay in the house and look after her cats and garden.
My other sister and her daughter have stated to me that he was living in his van and my sister gave him a room as they had become friends. Although they argued terribly and he was often drunk and abusive.
It looks to me that he may have persuaded her to renovate the house and build a large extension, although he claims to have paid for a lot of the work himself and that he undertook a lot of the work himself. I believe that my sister paid for the work.
My question is, would he have any claim over the house. As he is still living there does he have any rights to stay, what would happen if changed the locks and would not let us in.
He kept mentioning how much money he had spent on the house and then bursting into tears about his loss.
I realise this is very early days in the matter but my other sister has a history on mental illness and I want to prevent her from having a breakdown, so i want to be fore armed with the relevant information.