Welcome Worried Daughter,
I'm sorry to hear your mom is having to go through this - God bless her, at her age, for already beating cancer twice. She's done it before, she can do it again! Although I'm much, much younger than your mom, I too am petite (only 4'11") and before treatments weighed maybe 90 pounds soaking wet. I had surgery, chemo and radiation and all told I lost about 10 pounds. I agree with Gary - get a PEG tube for your mom. I had the tube inserted during my surgery and used it daily for my nutrition which is SO important during radiation. After 8 1/2 months of faithful use I was able to bid the feeding tube a fond adieu!
I'm glad you're able to be there for your mom - she'll need you more than ever. Keep strong.
Nancy