Sometimes questions like this are really, "I stole a domain name two years ago, and I want to know if I am safe from liability yet."
But, yes, by all means, the BEST lawyers are the ones who have a pat, certain, and simple answer to every conceivable question, regardless of how factually incomplete the question. <chuckle>
The problem is that in real life, things are very seldom cut-and-dried, or black-and-white. Real life situations are messy. It's not "what do I need to prove I had a domain two years ago", it is "this is what I have today, can you make a case out of it".
There is no legal fairy godmother who has decreed, "In order to prove you had a domain name two years ago, you need X, Y, and Z".
If the cause of action is "theft", then the statute may be two years in some states. However, no court has ever recognized a domain name as property subject to theft or conversion. That doesn't mean a court never will, but it would just make it harder.
The statute of limitations for contract-based actions can be much longer than that (e.g. five years in Delaware). If there had been some kind of contractual relationship - such as the common situation where a technical employee or outside tech contractor runs off with the company domain name - then you might have some grist for the contract mill there. Or if the registrar had made some kind of error under the domain name registration contract, you might have another angle. It depends on *what happened*.
But, yeah, the guy wants *the* correct answer to the question.... yah right. I guess we should just check the stone tablets which Jehovah engraved with the Rules For Domain Names.