Basically if you have any problems ie the house burns down, your normal buildings insurance won't cover it and when you change the buildings insurance to buy to let they will want to see proof that you have informed the mortgage company that it is buy to let.
If she's just letting out her house she would be fine, not even charged more for a certain amount of time, a couple of years i think, if she's bought a house with the sole intention of letting it then she's taking a big risk.