All you can say is exactly what happened to you. Such as "XX never paid me for a job I did for them".
I had a non paying client threaten to sue me for libel and slander once. I told him, please do. I then sued him a week later, and ended up getting a default judgment when he never answered the suit….as I knew he would.
Most people that say they're going to sue, never do. My response is always "go ahead". And libel/slander is extremely hard to sue for…any lawyer taking that case will require a lot of money upfront, and unless you have very deep pockets/insurance, that same lawyer is going to tell the guy to stop wasting his time.