...but it IS OP's fault. I don't get those saying it isn't.
It's not his responsibility to 'cat-proof' his house, it's the pet owner's responsibility to control their pets. As for his bird, he can let it fly around freely to its heart's content!
Put the shoe on the other foot and ask what you'd be saying if the neighbour had a pitbull and that pitbull had been let out into his garden. Now let's imagine that dog had escaped the garden, made it into your house, and killed your cat or even your child.
Do you think people would still be saying 'ah but it's not his fault'???
I'd eat humble pie. Do some research, find out what options you have for your cat AND him.
This is 100% the cat-owner's fault, so I'd start by looking at the possibility of keeping the cat indoors. If that's not possible (I'm a cat owner, so I see the issue), look at maybe buying him a new budgie, a proper lockable cage AND a dog flap on a timer so this doesn't happen again.
As I said, put yourself in HIS shoes... if this was your beloved pet, or your child, imagine THAT anger! Now think of all the steps you would want them to go through to prevent it from happening again.
I'm sorry, but if someone did that to my family... I wouldn't even want to think about what I'd be thinking...