What is the ruling on someone who filed a lawsuit against another person and falsely swore on the Mus'haf that the defendant insulted him, then retracted his statement before the judge and claimed forgetfulness, while the judge ruled the defendant innocent?

Your friend must repent from swearing falsely by Allah, for he has committed a great sin, which is false swearing, further aggravated by swearing on the Mushaf. The majority of scholars hold that no expiation is required for this, while some deem expiation obligatory for such an oath.