She may be afraid to file a complaint, worrying that it will only anger her abuser more and make the problem worse. This is a reasonable fear, but the abuser is more likely to re-offend if no action is taken. She may be reluctant to take action. Her abuser may be sorry for what happened and may treat her better than ever before - for a while. She may still love the abuser very much.
Many couples experience a happy, peaceful making-up period after an episode of violence.
As good as things may be in the honeymoon phase, the abuser's behavior seldom improves without help. She believes the abuse is her fault. She was raised to be passive and submissive. She developed survival skills instead of escape skills.