I encourage you to follow the advice you have been given to try to save your hen, but I also want to tell you my personal experience with ISA Browns. If you feel comfortable doing a home necropsy after your hen passes, I think you will learn that there was nothing you could have done to heal her. I've had well over 100 ISA Brown hens come and go over the years, and the emaciated condition you describe has always without exception proved to be the end stage of reproductive cancer. Like you describe, by the time they act sick and I pick them up, they are nothing but air and feathers. Your hen may have other reproductive issues like salpingtitis too, but the emaciated state you describe screams cancer. I am sorry. ISA Browns are so curious; vibrant and funny, and they lay beautiful big brown eggs. But when it comes to a long chicken life, they definitely draw the short straw.