Is a mother who refused to allow her son to marry, despite his consent and the approval of the girl and her family, considered unjust, especially given that her refusal was based on a single visit?

This world is a place of trial, and one must face it with patience to attain the good of this world and the Hereafter. One should not dwell excessively on something that has passed, for what Allah has diverted might be better, as He says: "And it may be that you dislike a thing while it is good for you." Increase your supplications that Allah may facilitate your marriage to someone better than him. As for the young man's mother, it does not appear that she wronged you by preventing her son from marrying. However, if she wronged you in some way, such as spreading false information, then it is permissible to supplicate against her, but forgiveness is better.