Has the client been treating the business as a S-corp since 2014 (which includes being on payroll, with all of the associated payroll forms)?

Revenue Procedure 2013-30 generally requires "reasonable cause for its failure to make the timely Election Under Subchapter S and has acted diligently to correct the mistake upon its discovery". Unless it has been treated as a corporation since 2014, I don't see how 2013-30 applies.

Besides, you would have needed to use that on 2553 that you sent in. It is not a year-by-year thing, if you thought that applied, you need to have already done that before you sent the 2553.