Abe, I'm so very sorry to hear about little Kiko. There is no way of knowing what caused Kiko's death unless she had been examined by a veterinarian and sometimes even then it can be difficult. Whatever happened, you had no control over the situation as you weren't there. You say your friend didn't see what happened and didn't see any marks on her.
It is so sad that little Kiko had such a short life. There is nothing that can be done about it now. I know you loved her and you gave her a good life during the time you had her with you. Whatever happened, she is now free of pain.