Is it permissible for a young man, who was harmed by his wife's concealment of her chronic illness from him, to file a lawsuit to reclaim his mahr of 25 mithqal of gold, after the court has granted a khulʿ? And would that be an injustice to the girl, or should he leave the matter to God?

If a repellent defect was found in the wife before marriage and was concealed from the husband, and he did not consent to it after the marriage, the husband has the right to annul the marriage. If the annulment occurs after consummation, she is entitled to the Mahr, and the husband can claim the Mahr back from the one who deceived him. If the guardian knew about the defect, he is liable. If he did not know, the wife is responsible. If the deception came from both the woman and her guardian, it is more likely that the guardian is liable.

What the judge said is correct, and you have the right to file a lawsuit against the woman's guardian to demand the Mahr. However, if you are patient and forgive, that is better.