It's a tough descision...what should I do?

I'm a manager at a local diner, and I'm having a little employee trouble. One girl is very hard working. She works up to 12 hours a day, and still manages to come in early and close late for others when they call in sick. This past week, she put in 50 hours in four and a half days! The customers all like her, and she never misses orders. She never complains or calls in sick. However, an employee who's worked with us for a year has some concerns that I'm finding hard to validate. She says the girl is rude, slow, and doesn't socialize. I've noticed this employee tends to give the hard worker a hard time, such as writing sloppy orders, or vague orders (ie, she'll write down breakfast. She doesn't write how the eggs are to be done, bacon or sausage, white or whole wheat toast...etc). I've also noticed that this employee has gone down hill. She'll sit in the diner, ordering others around while she takes long smoke breaks and does a crossword, all the while yelling in the back door at this hard worker that she's burning something. I've confronted her about this, as has the owner, but we think we have to replace the slacker. Why do you think she acts this way? Is she just immature? Should we replace the slacker and move on, or try talking to her again?