Need help quick!!! Cat hates new cat, what should I do to make her accept him?

They should be placed in separate rooms for the rest of his stay. Cats are often slow to adjust to changes in their environment, and I know when I brought mine home from the shelter, I was told to keep him in a separate room from other pets for at least a week to let him settle in and calm down, and allow contact only through a baby gate or sniffing under a closed door. Then following that, to confine the other pets while he explored the rest of my home, and allow supervised contact only for several weeks after that. I would not think it would be appropriate ever to just put two cats together, particularly when one has already established that your apartment is "hers". It would be unreasonable to expect them to get along with only a couple days to adjust, and cause more stress than it's worth.