I am so sorry to hear of your mother's diagnosis. I know how devastating it is. My husband was diagnosed with Stage 4 esophageal cancer with mets to the liver in Nov. 2004. He passed away at 59 in Nov. 2006. He took the same treatment being recommended for your Mom. It can be quite debilitating, but he fought the good fight. Your mother is almost 20 years older so it would depend on how she feels she could manage and whether the extra time gained would be worth it. My husband had to try anything because he couldn't swallow. If she is able to eat and lead a fairly normal life she might opt for pallitive care and forgo the side effects that literally end your life as you know it.
I hope I haven't been too negative, I am just offering my point of view. In the end, it is her decision. I will keep her and your entire family in my prayers. Please feel free to ask any questions.