What should a wife do who lied to her husband and attributed her past actions to her friend to cover herself, and now fears the sin of slander, and asks about how to please Allah and correct the mistake, and is she considered sinful if she cannot be frank with her husband and her friend?

You must continue to deny it, even if by lying, and ask Allah for well-being and concealment. This is based on what Malik narrated in Al-Muwatta from Zayd ibn Aslam that the Messenger of Allah, peace and blessings be upon him, said: "O people, it is time for you to cease violating the bounds of Allah. Whoever commits any of these abominable acts, let him conceal it with Allah's concealment. For whoever reveals his misdeeds to us, we will apply Allah's law upon him." And his saying, peace and blessings be upon him: "Allah does not conceal a servant in this world except that He will conceal him on the Day of Return."

As for falsely accusing your friend, it is forbidden. Allah Almighty said: (Indeed, those who accuse chaste women, unaware [of the evil] and believing – are cursed in this world and the Hereafter, and for them is a great punishment). You must perform sincere repentance with all its conditions.

In repentance from false accusation (Qadhf), it is not required to inform the slandered person. Rather, it is sufficient to speak well of her and supplicate for her. This is what Shaykh al-Islam Ibn Taymiyyah chose, because informing the slandered person is a pure mischief and may lead to enmity. However, you must still deny yourself to your husband, even by insinuation or lying, because the repentance of the slanderer is to deny their own accusation.