There's danger on both sides in this type of arrangement. Either party can start to take advantage, landlord starts to demand either more sex, or a cut of each booking, become controlling of who she sees threatening to change the locks and stop her working - or as posted above tenant decides to not engage in their end of the bargain. Who ever starts to shift the goal posts, it can then only end in disaster.
Best avoided. Blokes do text and email offering these sort of deals, an old member on here offered me free use of his flat if I were to tour London, in exchange for a special rate booking. There was no way I was going to take up that offer, he had a reputation of wheedling his way into WGs lives and causing them problems.