Can I tell me how I can I help my friend manage her celiac disease?

Educate yourself. She already has the resources to learn how to manage her illness, and she must do this on her own. This is seldom overly difficult. Often the onset of a lifelong illness is a lonely time -- no one likes being different. Being a good companion will go much farther than your offering lay advice from the internet. Be the member of your group who asks about gluten-free food options.