Amy - Have you heard of the book by Michael J. Bradley called "Yes, Your Teen Is Crazy" (or Loving Your Kid Without Losing Your Mind)? I gave it to my son when his stepson was going thru the teen years. Kids go through such dramatic physical and emotional changes, it's no wonder that life sometimes is difficult for them and those close to them. Probably similar to the cancer survivor who has to cope with so many changes and control issues and grieving over the loss of a previous way of life.
Liz - Getting the PEG and some fluids should really help Robin to feel better. My son did not have a PEG and went from 225 down to 183. After that he was very secretive about his weight but as soon as he could manage something besides water, his disposition improved greatly. Some liquids that he couldn't manage, he would have me dilute with water and they seemed to go down better that way. Hang in there and remember to take care of yourself, too! Things will get better.