First thing I would suggest is to get some crushed oyster shell and have it available at all times - they will eat it when they need it. That's the easiest way since it can stay out and won't go bad.
Other things we feed them that are high in calcium are yogurt, some cheese (small amount), collard greens, kale, and spinach.
We sometimes take a "bunch" of the collards or kale and hang it up in the run...so they get the entertainment factor as well. They'll strip collards and kale completely down to the stem.
Here's a link to the Oyster Shell we use from Tractor Supply:
Haha well thanks a lot now for the egg tip. Its funny that when the egg goes out is as healthy as when it goes back into her, its like recycling that calcium haha, goes in a circle lolNot at all. I just found it easier to scramble it lol. My hens issue was she didn't like the calcium and wouldn't eat egg shells on there own. Gotta bribe them and make it tasty like putting beets in Popsicles for kids hahahah.
I try to save all my egg shells and feed them back to the flock. It's like a little treat for them now. And I hardly have to fill there calcium bow
It depends somewhat on whether your hen has just recently started laying, too. Sometimes there doesn't have to be a thing missing in their diets. Early on, it takes a bit for the assembly line to get in sync. So rubber eggs, mis-shapen eggs, very small eggs, or even shell-less are not uncommon. I think we've all had them from time to time. If she is new to laying and you haven't been feeding layer food yet, now is the time to start.
Going on the assumption that she's been laying for awhile now and this is a new development, there are several things you can do to help harden those shells. I have a mixed flock - roos, and chicks of 3 different ages all integrated into the same coop/run, and it's just to difficult to teach them to only eat out of their own dishes. Yeah, that'll happen! So I feed an all-flock food and keep a separate container of oyster shell that the girls will nibble on if they need it. All of mine also love "hard boiled egg day"! I get out the oldest eggs in the fridge - those are still good but just a little too old for most of my customers' peace of mind - and boil 'em up, chop 'em up, and feed them to the chickens, shells and all.
Greens are also good, as has been mentioned. I don't mess with yogurt - my chickens never acquired a taste for it despite many attempts.