Is it permissible for a mother to conceal from the father something she discovered about their daughter, fearing it would ruin his life, and is it permissible to medically rectify this situation if the perpetrator does not marry her?

For the mother whose daughter has fallen into sin, she must deal with the matter with wisdom and insight. Firstly, by reviewing and holding herself accountable: Did she neglect her responsibility towards her daughter? If so, she must repent and rectify what remains. Secondly, she must guide her daughter by urging her towards piety and fear of Allah, and making her aware that her action is a grave crime against Allah, and that she must sincerely repent and seek forgiveness. Thirdly, the mother must conceal her daughter's matter, even from her father, based on the Prophet's (peace be upon him) saying: "Whoever conceals a Muslim, Allah will conceal him." Thus, not informing the father is considered an obligatory concealment. As for the question of correcting the situation, if the intention is hymen repair, that has already been answered.