Don't worry about the letter, I bet they have more to loose than you coz' they do all kind of  fake things, after all its a dhoti company. I bet they cofirmed your work history too and many others. Reply them with what you got on them... all the fake things that they do and the tax fraud and most importantly the USCIS..I'm 100% guarenteed that they will not go for law-suit. Its just a threat. Please mention the name of the Consulting Company if you don't mind.