It is hard to believe that this person is so believable that the social security administration just took his word for what you were doing, decided to charge you without an investigation, cut you off for 5 months and have declared you overpaid by $15,000. I wonder if there is something somewhere that was a little bit dirty looking when they did do an investigation?
When you go in front of the "law judge" what will you say? How will you respond? Where will they claim that you were working? How on earth do you think they got you overpaid simply on the ONE person's testimony? Did the employer confirm or deny that you were working for them? Keep in mind that if they wish to do so, they can check all work that has been done under your social security number, all wages reported paid in, and your federal income tax information. Are you completely sure they're going to see nothing,that this person's report that you were working is completely false?
Even if this is the case, which really isn't very likely, you still are not going to be able to sue him for slander or libel. Most of the places you can report people for disability fraud or any kind of fraud, there is a no fault or a guaranteed anonymity protection for those who report people. Did he tell you he sent the letter? Did they show the letter to you? Why are you so sure he was the one who reported you?