RSU is not a statutory option.  It is taxable upon vesting, which is exactly the information that was left out of your question.  What you should look for is the amount that was already reported on the W-2 as compensation in the year the RSUs were vested and delivered in stock.

There is nothing Amazon will need to report in the years subsequent to the vesting, whether or not the former employee sells or retains the stocks.

I suppose the stocks were still held through the custodian broker, which would have records of the basis and they would have reported that to the IRS on the 1099-B.  The way you report these transactions on Sch D would be no different from any other regular shares your client bought from the open markets.