Thank you. So I assume then that your question is whether you can sue the employer or the employee.
In regard to suing the employer, that would typically not be possible due to the rule in CA that employment is "at will" absent an agreement to the contrary. At will employment can be terminated (or rescinded before it even starts) for any reason not amounting to discrimination on the basis of a legally protected trait (race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination is fair, reasonable or even TRUE. So, the fact that this employer rescinded their offer based on a lie would not make their decision illegal, and you would not typically have any claim against them.
You would in theory have a claim for defamation against the employee, though. The problem is that you would bear the burden of proving that their statement was actually false. I don't know the underlying facts here but in general it is very hard to prove that something years back didn't happen. Unless you have a very clear way of proving it then you might not be able to meet your burden of proof in a defamation suit. The other thing to consider is that defamation suits take time, and you will probably need to spend an attorney at least $5,000 to get it started. Then, you may wind up with a judgment against this person but they could have no way of paying it. This, sadly, is why defamation suits against individuals are rarely brought unless the individual is known to be wealthy with the ability to pay a judgment. Otherwise, it just doesn't make practical sense to pursue such a claim.
If I were you I would focus my efforts on trying to convince the employer to reconsider their decision rather than suing the employee. The latter is likely to be a huge waste of your resources that could better be spent trying to salvage this job opportunity or look for another job. This is the honest truth.
If I can clarify anything at all for you, please do not hesitate to ask. It is my pleasure to assist you further if necessary....