I find it funny that a user named 'PoisonApple' is asking about morals. :D
Anyway, your question. You don't like her, is she 'an enemy' or just someone you personally find annoying but neither of you are actually fighting? While not everyone believes in Tarot you might want to tell her, or at least suggest she should study harder, so you at least tried to warn her, so if the reading comes true you won't feel bad you did nothing. The fact you asked the question says you would feel bad doing nothing. In the future you might wish to do readings upon request only to prevent these moral dilemmas.