There is nothing wrong with feeding eggshells unless it results in the hens not eating enough other forms of limestone. All hens have a reserve of calcium that they use to make eggshells.
This calcium reserve is their own bones, think of it like a hens' bones are her calcium ATM. The calcium...
I would nix the eggshells and only give her oyster shell. While eggshells are a good source of calcium (at least in theory) the particles of crushed eggshells are so minute that they don't remain in a hen's digestive track long enough to be adsorbed. It's like trying to add water from a fire...