Are you sure this was not a 409(a) plan? A Non-qualified deferred compensation plan would only make this taxable when it is distributed. I find it interesting that you have a starting point in 2017, since the TCJA changed the treatment and that is the same timing. Or a 457(f)?

https://www.thetaxadviser.com/issues/2016/aug/treatment-for-nonqualified-deferred-compensation.html

Nothing would be reported as wages/compensation, unless it is reported on a W2. A tax preparer would only report wages if there is the belief the employer didn't handle their reporting requirements properly. I think the point is to find the factual info and not keep making assumptions. You're going to need a better resource than this employee and their own communication and forms from their employers.