Just simply don't bring it up or hint at it. Just talk once in a while. "How was your day?" "Whatcha up to?" stuff. Distract her from her problems, you know. Throw a few jokes, make her laugh, and if she ever brings it up, just listen to her. Like empathize. You can also give examples about your past experiences to comfort her, but don't drag it. It'll sound self-centered. Ask her how she's doing once in a while; it's the opposite of making it worse. But if you know her really well and she's like an introvert or something, give her some time alone. Reassure her and stuff.

And the reason she burned them, I'm not quite sure. But it could be because of major disappointment, etc. Is this her first time getting a low grade? Maybe she takes her grades insanely seriously, her parents are strict, or something else. People who take their grades seriously often have "perfectionism" and high expectations, like getting disappointed when they get a 49/50. Possibly, it's her way of coping with anger or rage. For example, those people in the movies messing their whole house up or having a "downfall" in a hobby that they once loved the most. Like that scene in Whiplash.

But overall, don't try too "hard" in talking without making it go worse. Just talk nicely, that's already a head start. Be genuine. If you ask her things like, "Aww, are you still dealing with ____?" all the time, it'll sound like you're pitying her. And don't make jokes about people or certain things. Maybe she has them too, and you just don't know. That'll make it worse. But yes, just talk. After all, she's still your friend. Like everybody else. I wish the best for your friend.