So Skinny - how mch weight can he lose???

My Dad has been struggling through stage 4 NSCLC for 18 months. He is 6'+ tall and weighed 206 pre-diagnosis. He is down to 133. How thin can he get? I know this is an odd question, he is just so frail, how can he go on. He has actually been eating more lately, I couldn't believe that he has continued to lose weight at such a rapid rate. It has been about 10 pounds this month alone with basically no activity? How thin does a patient get when they go through this???