“Hey, this situation has been difficult for me. I enjoy working with both you and Lazy, and I miss the days when everyone got along. I don’t want to invalidate your frustrations, but it’s stressfull to hear about how lazy Lazy is being. It doesn’t bother me that Lazy does less work, but the tention between you two does bother me because I enjoy both of your company”
And see how Hardworker responds. Then go to Lazy.
“Hey Lazy. Hardworker seems upset that you take more breaks than they do. Do you think you could talk to her and try to clear the air? I enjoy working with both of you, but the tension between you two is upsetting”
And again, see how they respond.
It’s entirely possible that neither of them will respond well to this, so bare that in mind. If it’s stressing you out, though, you have an obligation to yourself to try. Just also be aware that it could backfire. So be sure to weigh whether or not it’s worth it to get involved