Mom diagnosed with Lung Cancer. Need advice on helping her to eat

Hi, I'm pretty new to this group but I've seen the support and help you all give each other and I think it's great. I"m writing today because my mother was diagnosed with lung cancer. We are being seen by an oncologist and receiving treatment. Where my sister and I need help is with getting her to eat. to make a long story short, my mother was doing okay until recently when the docs found an arrhythmia and prescribed a blood thinner. Well my mother ate her self up with worry about taking the medicine. She worried so much and she lost a lot of weight. Having lung cancer and losing weight is not a good combination and my problem now is she only eats about 700-900 calories a day. Not enough to gain any weight and certainly not enough sustain yourself. She has always been a petite person but now she is so thin, my sister and are afraid. She doesn't have enough energy to do anything, she's constantly so tired. we try to tell her it's because of lack of nutrition. My sister and I are trying everything to get her to eat enough food in a day. My question is, has anyone experienced this? or have loved one who has gone through this? do you have any suggestions on how to get them eat? open their appetites? what kinds of food might work? I appreciate any thoughts and ideas. My sister and I are at out of our minds with worry. Thank you in advance for your support.