How do I stop this?

[Hairyhat]

One of my chickens keeps eating the eggs. She seems to enjoy the shell most. It makes a horrible mess in the straw. I've tried letting them out at different times but I'm mostly out when they lay

Making sure you remove the eggs as soon as they’re laid will help massively. Failing that (as we all know we aren’t always here 24/7!!), make sure whatever you’re feeding them gives them plenty of protein and make sure they’ve got a good supply of mixed grit. Usually they start eating eggs because they’re searching for the protein and calcium that an egg contains, but it can develop into a habit over time. Feeding a good quality layers pellet/mash and plenty of mixed grit should be enough for them, you can add poultry conditioner if you think they need it. You can also try leaving golf balls (or specialist shops will sell you a rubber or ceramic egg for ££) in the nest boxes which should put them off when they work out its solid and don’t get what they want. Old fashioned remedy would be to blow out the contents of an egg, replace them with mustard/Tabasco/curry paste/anything unpleasant and strong tasting and replace into the nest box. That would put them off once they try to eat that. If all of those fail you are likely going to have to look at identifying and culling the culprit or you’ll risk them all starting to copy when they work out what one is getting that they aren’t.

Thank you @NoParticularPattern that's a huge help! I know who the culprit is as I only have 4 hens but I'm not going to cull her. She's the smallest and a bit henpecked so I'll make sure I up her feed and try the false egg idea!