I am so sorry for your loss. I also feel for your freelancer.
I have had this with one I just bought, it was awful, I was in shock as it was so unexpected (passed 5 stage vetting 24 hours previously!). It was not the most pleasant thing to witness either, I feel for your freelancer. She must know she did nothing wrong, but it is difficult not to blame yourself.
I had that one PMed, it was simply an electrical heart attack, no rupture, nothing. I was out with a friend years ago, theirs ruptured an aorta. After mine, my vet said it is more common than people think.
How awful for you and her rider. I think she will not have suffered and I am so glad you could have her brought home for her friend to do the thing they do and nobody was hurt. Three small positives to try and ease what must have been a dreadful day for you. RIP Fizz