Let's look at the bigger picture here. Is there a chance the eggs are breaking when being laid more often than normal? Make sure there is ample oyster shell available, and put some padding in the nestbox if it's bare wood under shavings. I get 2'x2' carpet squares at the home improvement store and cut them in four to line nests under the bedding.
Do they have ample clean water?
What do they eat in a typical day? You said adding mealworms helped for two days. What else changed? If they're getting a 16% feed and they also get vegetables/fruit/scratch in a day, that would mostly likely be your culprit. I've read many people say the sexlinks tend more strongly towards egg eating, I would assume their higher rate of lay makes them want a bit more protein than is typically offered.
If I were in your situation, and you KNOW you know which one is doing it, I'd pen her by herself for a few days with a much higher protein feed and a padded single nestbox. You can pick up a bag of gamebird feed - that's usually 24-28% protein, and feed her that alone for a couple days, and then mix it in with your regular to bring your protein up overall a bit.
I wouldn't faff with making a mustard egg. I know some people have said that it works for them, but I've seen it so many times on various FB groups that it doesn't work for most, and I don't see how it really would as chickens aren't bothered by spicy food.
