In the movie The Client, the last scene shows Han confessing to how he took out the body of his murdered wife. He throws it out of the balcony. Up until this moment only Kang has been talking and saying stuff and mostly speculating. We can assume that the only confession that Kang has recorded that he gives to Ahn is about taking out the murdered body of the wife by Han. But Han has just been tried for the murder of his wife and the verdict was "not guilty". The evidence that Kang has recorded and the finding of the dead body of the wife all points back to evidence of the same case. As stated in the movie earlier, even if the body shows up later Han can't be tried for the same case because of Double Jepoardy. Han has not revealed anything about murdering the school girl before, so there is no evidence for that either. What's the point of arresting Han? They still have nothing on him. Wouldn't Double Jeopardy set him free?
Double jeopardy means you cannot be tried twice for the same crime. Usually in a serious crime, like murder, there are other associated crimes that suspects are not charged with, because they can be harder to prosecute and won't result in as great a sentence on conviction.
In this case, Han could possibly be tried for obstruction of justice, perjury (if he testified or gave an affidavit denying guilt), and other felonies depending on the jurisdiction and the nature of the murder itself (e.g., improper use of a firearm or assault with a deadly weapon, perhaps). I believe there are improper burial, failure to report, and giving a false statement to police crimes in many jurisdictions also.