From what I have read, everyone has a different story with melanoma. It seems there is no deffinate answer to what will happen step by step. It is a beast that just does its own attacking....
My mother had melanoma removed 12 years ago, and had yearly checkups that were fine.
May 2005 she was diagnosed with stage IV. Shocking to us, because we never knew it returned.
Had a tumor in the brain, and then found cancer cells in the spine in May 2006...that is when they said chemo, so she did. but June 2006 they said no change. Once it hit her spine, then they said 2-4 months.....she passed away about 3 weeks after they said that....really fast!
So, I wish I could help you with a step by step picture, but I can just say try to be as supportive as possible, she will need care during chemo.
best of luck, and my prayers to you.