how to get cat to not eat our bunny

I have a cat, her name is mouse. She doesn't kill things, except for mice and any type of bug she finds. However, I had a bunny about a year ago (it was wild, I saved it) and she was "playing" with it and killed it. 
We are getting a potty trained bunny this weekend so it will not be in a cage, I'm scared that I will have to keep them in separate rooms. I don't want my cat to scared him or kill him. The owners said he loves other animals but our cat on the other hand doesn't do well with other animals. She won't even let anyone near her but me. If we don't get the bunny it will be killed, that's why we decided to take him. When the last bunny was killed, we had 4 cats. She was playing with it but I don't actually know which on killed it.