Most Helpful Girl
You can't! Hearts mend themselves when the person with the broken heart finds a way to let go of the pain/ hurt.
Most Helpful Guy
You can't 'unbreak' someone's heart, but you can help mend it with time and effort. Trust is broken and confidence is destroyed, so the only thing one can do is apologize and show them you're willing to do what you can to help them. Actions speak better than words, so if you want them to be doing better, you need to show them.