She may not be (is probably not) lacking. Calcium "problems" including both lack and excess often appear similar. She may be lacking in other minerals that help in the even distribution or she might even have excess. If you are feeding layer pellets or mash she should be getting plenty - you probably don't need the shell grit as well - so long as they have access to plenty of grit to help with their digestion. Not that it will hurt, just you probably don't need it. You can check by looking at the mineral distribution on the bag of pellets - they are generally created for comercial operations where there is no other food sources.
I have always fed their shells back to them. Many people feel that it leads to a tendency for them to eat their own eggs - but I have never found this to occur. They will eat an egg if it cracks or breaks (or if a crow cracks or breaks it for them) but unless they have mineral deficiencies they genearally don't even look at their eggs as a food source.
