Wow I’m late to this conversation. Sorry this is happening Michelle. I have had a hen into the vet for a shell less egg and the vet said there is absolutely nothing you can do...medical treatment wise. He said if it happens a bunch of times then you know what problem you have (repro probs).
When they don’t lay a normal egg, they dont feel well at all, before and after. Although they feel better after. I don’t think you need to separate her out.
Leghorns are high production birds, I think she is not getting enough calcium with your all flock for her. Being high production.
A suggestion, try setting out some crushed eggshells (feed them back their eggshells) in a couple of places (where D can get at them without being chased off from the new novelty), and see if things improve. Wash them, dry them out good and crush them up to very bite sized pieces. A lot of birds just do not enjoy the oyster shell. Also make sure she is eating the bird feed vs treats that aren’t feed (I know you are).