Aw sorry to hear about Lucy's health issues. When my babies get sick, I feed them chicken as well. They get the same volume as what they usually get in kibble or a little more.
Boiled chicken breast contains a lot of water, so I found that they always seemed hungry still afterwards (which is great because they had an appetite despite being sick!). I try not to give them too much though because usually when they are sick and on the chicken / rice diet, their GI system needs to take a break anyway. I prefer to give them small meals throughout the day.
Oh and, if you're unfamiliar with the boiled chicken diet, I found that when they eat this, they poop much less and their poop is smaller. So don't be alarmed if she doesn't poop one day.
My dogs LOVE to eat poop, so I can sympathize. I never walk them without a leash outside. But even then, they manage to eat something bad somewhere, somehow.
Hope Lucy recovers soon!