In three days my friend is dropping off his cat so I can watch it for a week while he is on vacation. However, the last time I watched his cat my cat attacked him. These two cats hate each other and they drive me crazy! How can I encourage them to get along for a week?
Your cat is seeing your friend's cat as an intruder in his territory. It's natural for him to try to drive the intruder out.
I'd suggest the standard method: shut your friend's cat in a room with everything he needs and visit during the day. It's better if the room is one your cat doesn't spend much time in but not essential.
Generally after a few days your cat will have relaxed enough to accept the visitor, but if not, your friend's cat still has a safe place and is getting food, water and attention until your friend returns.
You might also look at some of the answers to this question: How to get cats to coexist in peace?