How can she forgive him for hiding something so important from her? How can he forgive her for talking that way in front of his parents? Romantic partners, friends, children, parents, teachers, colleagues, bosses, employees - anyone can make a mistake that requires an apology before there is any hope of healing the relationship.
Insults and hurtful behaviors can have such serious repercussions. The pain of betrayal and loss of trust is excruciating. The humiliation and shame can be debilitating. How can they think that "I'm sorry" could ever hope to put the pieces back together again?
In fact "I'm sorry" alone is insufficient. It has to be a sincere and meaningful apology. There is a way to determine to what extent the apologizer really means it. That doesn't mean you will necessarily forgive the person who hurt you. But it is necessary before there is any thought of reconciliation.