Yes, correction is needed even if the participant elects to not defer going forward.
You are close. The sections you need in Rev. Proc. 2016-51 are Appendix A.05 (9)(b) and Appendix B 2.02(1)(a)(ii). When you put them together, I think you will find that the pre-approved correction method is for the employer to deposit a QNEC of 25% of the applicable missed deferral rate times the participant's compensation for the portion of the year he was excluded, plus lost income and 100% of the missed match for the portion of the year he was excluded based on the applicable missed deferral rate. Note that a notice to the participant is one of the requirements to be eligible for the 25% rate.
You don't say if the plan is safe harbor or not. If it is, the applicable missed deferral rate will be based on the SH formula instead of the NHCE average deferral rate.
If you are looking for a summary that is easier to read than the Rev. Proc., try this page on the IRS website:
https://www.irs.gov/retirement-plans/401k-plan-fix-it-guide-eligible-employees-were-not-given-the-opportunity-to-make-an-elective-deferral-election-excluding-eligible-employees
The conditions for the reduced 25% rate are listed in the Corrective Action section of the page. Note, they add a requirement that the employee be employed at the time of the correction, which I don't see specifically mentioned in the Rev. Proc.