Friend in rehab is experiencing nausea and vomiting and can't gain weight after surgery. Her body healing, just can't eat. I suspect an eating order.

This is likely due to the pain medication being dispensed to your friend, not an "eating disorder resurfacing". This exact thing happened to my father in rehab.....he couldn't eat, was vomiting and nauseous all the time, and the culprit was Vicodin. As soon as it was discontinued, dad was fine and started eating again. Pain meds notoriously change taste buds and cause all sorts of stomach issues.

Discuss with friend's family or POA. This is really in their hands, not yours. Your discussing your friend's history may be a violation of her privacy rights, but as you are not in medical care you are not bound by any laws governing privacy. Were I truly worried I guess I would slip into innocent conversation with the POA or next of kin managing care, or the RN "Golly, Elma has lost a lot of weight. This worries me due to her history of eating disorder". They may well say "WHAT disorder" and you can say "Oh, sorry. That just slipped out. I am worried about Elma. I would ask her doc for a nutrition consult, but I must leave that in your good hands".

Up to them what they do with that.
If "Elma" as I have named her, is up to it you can TELL her you are going to drop that bomb on the POA. Or not. Or you can ask her yourself and not tell anyone else. Quite honestly, you aren't a family member. This really isn't at all in your control.

It may be the hospital food. Never very appetizing. As a friend all you can do is talk to discharge. Tell them your friend has no help at home. Rehab maybe recommended after 3 days, I think, of a hospital stay. "In home" can be ordered too. Does the woman have no family? Does she have you on HIPPA paperwork as someone the hospital can talk to? Do you have POA? If there are family members, then they need to inform discharge there is no one to care for the woman.

I would not be the one who picks her up if you don't feel she is ready to come home. Once you walk out that door with her, you will be responsible for her care.

I worked for Township subsidized Visiting Nurses. We saw clients only under a doctor order. We always were getting calls from residents all upset because they had no idea what was going to happen with a LO after a hospital stay. I had to tell them that discharge will discuss that with them. Rehab is usually a given, "in home" next. We could not get involved if another agency was already set up. If "in home" is not an option, then discharge needs to be made aware.

"Who to consult in Pennsylvania fo this elder problem?"

I'm not sure if you have the power to do anything if you aren't her PoA or Medical Representative. You can try to pass on this info to her primary doctor or the nurse in her rehab.

If she's so weak she can't stand, then discuss this "unsafe discharge" with the rehab admins. Again, if she's competent, she can check herself out if she can get herself home. Don't tell the staff you are her caregiver or they will think you'll be taking care of her at home, regardless of her problems you listed.

First and foremost I would make sure she gets checked to make absolutely sure she isn't having a different problem than what you think.