My dad and cat hated each other and now my cat isn't afraid to be in the same room as him suddenly. I'm confused by this and want to know.
Yes (obviously), but it depends on what you mean by "trust". It looks like your dad is no longer triggering the "fight or flight" response in your cat. This may be because your dad has changed his behaviour in some (perhaps subtle) way (by giving up trying to be friends, for example), or your cat may simply have become habituated to your dad's behaviour. Cats and dads tend to be creatures of habit. They may yet become the best of friends. Who knows? Give it time.