How do I comfort my friend who just found out her teenage daughter has cancer?

Listen and support. Listen to your friend and be there for support. She may want to talk about the diagnosis and may not. Do not prod for information- let her share as much as she wants. Offer to run errands, do the grocery shopping or clean her house. Let her know you are there for her in whatever capacity she needs at the moment.
COmpassion. Reassure her that cancer if discovered in early stage has a better prognosis than the one with metastasis.There is a lot of treatment options like chemo or radiation depending on what kind of cancer she has. Prayers help a lot!