There is a strong genetic factor, but some people with the genes never develop narcolepsy. This means there are other factors that likely help trigger it. Whether these are certain types of illness, environmental factors, or different types of stresses - all these things have not been worked out. It is likely that the triggering combination may vary from person to person. It is probably not possible to even say right now if her narcolepsy will even worsen or not.
Some people have a milder form that remains pretty much the same.
She should find a really good sleep specialist or neurologist that will follow her and work with her. I hope this helps. It is good she was diagnosed early. With a good doctor and proper medications she can have a good life. Many people with narcolepsy are not so fortunate and the disorder has robbed them of the best things from the best years of their life.