Is there an expiation or a way for a mother to rectify her mistake after burying her son, who died thirty years ago, in a wooden coffin due to her ignorance of Sharia rulings?

It is disliked to bury the deceased in a coffin, as it goes against the guidance of the Prophet, peace and blessings be upon him, and resembles non-Muslims in normal circumstances. However, if there is a necessity, such as fear of the grave being desecrated, then it is not disliked; rather, it is preferable in order to protect the deceased. There is no expiation (kaffarah (definition)) upon your mother, and what she did does not reach the level of being unlawful (haram (definition)).