She should eat a balanced diet, not just vegetables. Eating well and exercising (but not overexercising) may help regulate her cycle, but more likely the irregularity is hormonally-driven and not under her control. Still, charting it can help her figure out what's going on with her body. For example, she'll learn how to detect and generally predict ovulation, which will help with trying to conceive. Also, if she ends up needing to see a doctor about this, the charts will help him/her as well in diagnosing any possible issues.