My rat is extremely unfriendly with other rats, and I want to get a second one. Help!?

Its a sign of dominance. Animals living in the wild or not will fight to assert dominance. Your rat is showing your friends rat who is the dominant female.Just keep an eye on them and let them have it out. The sooner the better. If any blood gets drawn separate immediately and try again. She'll learn that rough housing is wrong and chill out soon enough. Tap her on the nose((gently)) when she starts to attack and tell her no in a firm voice. Its a slow process and you have to introduce them slowly and keep supervision on them to make sure they get along. They are like children and just slowly introduce them to one another. Give them time.