I'm so sorry for you! It's hard when they're under the care of someone else and something like this happens. But I'm sure your ex was not being negligent; it's just a terrible accident, and maybe could have happened almost anytime he was outdoors.
A friend here had a coyote in her back yard and she's got a fence (wall actually) around her back yard; the coyote jumped over it, which I didn't realize they could do
.
I'm sure your ex feels terrible about it too, but my heart goes out to you.