Is a wife's discovery that her husband had been talking to girls before their marriage, and her subsequent decline in worship after marriage, a punishment from Allah for her? And what should she do about it?

As long as your husband is righteous and upright, think well of him and live with him in kindness, and do not spy on him. If he committed sins before marriage, it appears that he has repented from them, and repentance erases what came before it. Thank Allah for the blessing of a righteous husband, beware of the whispers of Satan, seek Allah's help, and cooperate in good deeds, increasing in the remembrance of Allah and supplication.