You are missing the point.  RSU is not a statutory plan, which means there is no preferential tax treatment and there is no such thing as disqualifying disposition of RSU.

Income for RSU should have been fully accounted for on the W-2 and established a cost basis.  You need to refer to the grant/award statements (with reference to your client's compensation breakdown) for details of the cost basis and should report the sale as LTCG.

Even if it were a statutory option, which it is not, your understanding is not the correct tax treatment for disqualifying disposition.