Maybe the work mate is like me and got burned before?
I was the idiot that swapped the unsocial shifts and worked all the big holidays assuming that my work 'mates' would do the same for me. I was the plank that everyone assumed if they needed to swap, or needed last minute cover that I would step in
Until my Father was diagnosed with a terminal illness and I booked Christmas off (having worked the previous 10, no that's not a typo, 10 years in a row I worked some part of Christmas Day.)
We were told in the Sept that he had 6 months, so I approached my managers and told them I would be booking it off as I was aware no one else had and that it was first come first served.
Despite telling all the team that I was taking Christmas off several of them made plans assuming that I could be persuaded to swap.
When I refused I then spent the whole of November and the 3 weeks of December being told I was unreasonable, I was so selfish, that me having the time off meant that X/Y/Z didn't get to spend Christmas with their kids, never mind that none of them had ever worked on Christmas, and the amount of times I'd helped them out. This was with them knowing that this would be my Fathers last Christmas.
It was one of the reasons that I left the job I loved and that I was very good at 6 months later.
So it might seem like a 'little' thing here and now but you don't know what else is going on in her life, and as said above you knew that this was coming, its not an emergency situation.
Not unreasonable to ask but very unreasonable to comment once she said no.