The only possible thing i can say that might explain it at all, is that employers are theoretically allowed to decide when you can take your annual leave- i.e. it is dependant on business needs.
So if it was a particularly busy or important time of year when they cant really do without her, they could have declined her leave request straight out.
In which case, it could possibly be that they are trying to find a way of accommodating her request seeing as it is her honeymoon, by getting her to get everything up to speed before she goes.
I'm not saying that is the case, nor that it is right that they are doing it, just trying to point out a possibility. I have myself been asked a few times, to finish certain things before i go away as a condition for me to be allowed to take particular days off.
So worth trying to establish with them why they want her to do this, and if work needs are the reason, then trying to come to some mutually acceptable solution.