These things are never easy.... but telling her straight forward is the only way to handle it. I am sure if you try to hide this from her for even a while, she will take it harder when she does find out. I have been around cattle all my life, and I still find it hard when you lose an animal that is special. I do think that this same issue is why most agricultural based people who work with and raise animals, have a better balanced prospective about life, death, relationships, and lots of other things that are important in our lives, than some of our urban friends do.
My dad is now 83 and still helps me on the farm. I have only seen this man cry twice in my life. The first time was shortly after my mother died. The second time was two springs ago, when we lost a calf at birth. It was a normal enough calving, however the calf just didn't want to breath properly. It would take a few breaths and then stop. Dad and I worked with it trying to keep it breathing, but eventually it quit entirely and died. My dad sat down on a bale and cried . I have no idea how many thousand calves dad has seen born, or how many he has seen die, but it showed me that this death thing is never easy.
Good luck with your daughter. Honesty is always best. I am sure she will appreciate this, in time, and you will as well