i think that is because she would own half of the property as a joint tenant and people who own property have to pay out for care.
If they are still joint tenants, then his half of the property woudl pass on his death to the mother and the state would then get its claws on that too. I would suggest that he sees a solicitor to sever the joint tenancy so they become "tenants in common" and that he ALSO makes a Will leaving the property to the intended beneficiary (making sure he has a right to live there for the rest of his life), this should ensure that the state can't get their hands on it unless the father is then also put into care and his share is used bythe state.
There is a technical way around this with having the house put into trust/transferred to beneficiaries now before he dies but that must be discussed thoroughly with a solicitor to see if it is appropriate in these circumstances