My little sister has Crohn's also. She was finally diagnosed at around 22 or so (she's umm...39 now). She has had portions of her intestines removed, but, leads a very normal life. She does have to watch what she eats (spicy foods especially), and she rarely drinks alcohol, as it also upsets her Crohn's. She travel's overseas extensively and lived in China for the last 2 years, so she's probably eaten just about any type of food there is.
On the plus side, she stays slim without much effort, while the rest of us have overweight problems.
Your sister should research Crohn's and get a doctor who is experienced with it. With help, she should be able to manager the disease, and lead a long productive life.
We'll keep her in our prayers :-)