Strictly speaking this is a change in the terms of her employment. She should have some consulation, which is whats going on now and she can accept of refuse the new terms.
If she accepts she has to accept whatever pay they are offering for the post. If she refuses she migt be able to claim redundancy.
Neither are what you want to hear and kits unfortunate, but for them to begin breakfasts, they need to find a way round having the pub cleaned earlier. Maybe she can see another way round it and suggest it to them, but I can't think what else they can do really. Of course it would be nice if they offered her double time for working in the early hours - maybe she could ask for more pay if the breakfasts take off.