Giving calcium to a hen is like giving her food or water. She needs these things regardless of her current health crisis, so no harm will be done if calcium isn't one of her issues. If it is, calcium can save her life if she is experiencing a deficiency. Calcium is needed for bones and feathers and making egg shells, but it's also crucial to keep her heart beating.
Just this past week we had a hen that was egg bound die immediately after resolving an egg binding episode. In such cases, a calcium deficiency can cause the egg binding, but all the available blood calcium goes to making a shell so the egg can be expelled while none is left to keep the heart working. Early intervention with calcium is simple, safe, and may save a life, preventing a heart attack in the hen struggling with the stuck egg.