So, my friend has been eating nonstop lately, and I don't know if she is depressed or diabetic?

Many causes. While you're correct, there are many other possible causes for your friend to be eating non-stop lately. I recommend you start by asking her. She may have started a medication with increased appetite as a side effect. If she doesn't know, she may want to start with a medical evaluation if this has been going on for longer than 2 weeks.
Discuss with her. It is difficult to know why this is happening without talking to her. A lot things could cause her to eat.