The fact they have a relationship, or had one and she does have rights to live there, and in most cases standard landlord/tenant laws apply unless family court intervenes.
You have a right when you establish residency in a dwelling, it really does not matter what relationship they have. She does not have to rent the place, or pay rent for that matter. Legally she has the right to be there, and he cannot just throw her out without an official eviction notice.
There are legal ways to remove someone from a dwelling. In most cases you just cannot change the locks or kick her out. She can always call the police and if she lives there and he has not gone through the legal process to remove her, she has rights to live there.
With that said, it does no good prolonging the inevitable. She needs to start looking for another place to live if the relationship has deteriorated.