Sweetheart, my mom had advanced breast cancer and she overcame it. My grandmother overcome it twice (back in the 90s!). 

I was a similar age when I heard about my mom and I remember that cold feeling in my stomach. It was horrible. 

One thing I would raise - look into the side effects of chemotherapy, and ask her doctors how they are accounting for these side effects so they won't damage her long term. Chemotherapy is brutal and it's often overlooked that if not done with utmost care, people could suffer from after effects years after the cancer has gone. So make sure that they're aware of this and taking account of it. 

That should go without saying, but in our case, it didn't. 

When my mom had it - we tried to have some fun with it. We got colourful wigs, figured out recipes together, spent time together. Do the same for your mom and you'll find that you grow even closer because of this horrible experience.