My friend acts retarded all the alot?

Well I have this friend who is called Cathy and she's always acting so immature and jumping around and it annoys me so much! She acts like a 5 year old and we're 18. She doesn't do any work and have sex and sleep all the time in class. Also she likes elephants and I hate elephants... WHY IS SHE SO ANNOYING? And what's wrong with her?