Many foods are chicken-dependent ... some chickens love citrus and others just won't touch it. There aren't too many foods that are no-no's for health reasons. For example, no onions and peppers are generally advised because it could impact the taste of the eggs, not because of a health issue. My chickens have occasionally had both (cooked not raw) and we didn't notice anything.
That being said ... my friend gave her chickens avocado and one of her birds developed bad diarrhea. It took a while to clear up and it was a mess. I don't know if any of her other birds ate the avocado and didn't get sick or if that was the only one who ate it. In any case, I never give my chickens avocado. Others on this site may have a different experience.
If you are concerned, you might want to give her a list of what is okay. My neighbor occasionally takes care of my chickens when I travel and I have typed up instructions for her for letting them out, collecting eggs, locking them up, etc. Here's what I wrote for food: "You can bring them kitchen scraps dairy, fruit/vegetables, meat, bread. (No avocado, onions, garlic, or peppers. They dont like citrus or bananas)."
I actually don't know if they don't like citrus or bananas. I have never given them any. But since I haven't, I wouldn't want her to give them something that ends up having a bad reaction and then I'm not there to deal with it. Or, even if I was home, she would feel really bad about giving them something that made them sick so I just say not to do it.
Hope this helps ...