You should tell her the truth, after the party. It was extremely frustrating to me as a child than knowing I was being "shielded" from the truth that adults thought I was "too young" to understand. It was never my parents doing this, by the way, just well-meaning patronizing strangers. Grrrrrrr.

Any way, it'll be hard, but you should tell her the truth. Then stress that she should treat her ffriend as normally as possible, that her friend is probably sad at being sick and just wants to be a normal kid. Her friend will treasure a friend who sticks by her, as children who are ill are often shunned, sadly.