This is my first year with chickens and I've read and learned so much. One thing I read is to avoid exposed insulation in the chicken coop. I read one possibility is to place hardware cloth over the insulation. Some folks use bricks of straw as insulation or layers of recycled wool blankets.
As for health concerns, I haven't come across anything specific to styrofoam, but I'm hopeful it'll pass through without any problems. It doesn't tangle, which is good news for the crop, and it's lightweight and very well could get pushed along by other content.
When my hen ate a rubber band, I was really worried for her. But the kind folks in the forum told me most things chickens eat pass right on through, and that's just what happened.
It's the quantity of styrofoam that could be a concern, so hopefully it was shared among several hens. Maybe if you give them baths you'll find they're a little extra buoyant. Who knows!