not sure. But its only a month That's 2 saturdays and 4 sundays.
Though I fell your frustration and agree that some compnies do take the pees, if its only for the month of Jan. can she not do it and enjoy the days off in Dec?
On the flip side, she could check her contract which may state, in small print, that she may be asked to work during days/hours not part of her normal working week. So they could use this against you. If there is nothing like this in her contract then she could use that to her advantage.
What some companies dont realise is people have a life outside work.