I am a structural engineer and the building requirements are extremely strict , If I chose to ignore the guidelines and rules and a building I worked on collapsed then I would be liable .
The zoo has a responsibility to ensure the safety of visitors and failed to do so by not having a secure enclosure , just like if your dog gets onto the street and bites someone you are responsible for public safety too.
No lawyer in the world is going to be able to lay blame anywhere but at the zoos door.