How did you find this client?  Here's a hint.  Don't look for new clients there anymore.

Other countries manage to match payroll reports with tax returns, at time of filing.   IRS is sort of Third World when it comes to this, especially with having to rely on Social Security to collect the data.  But you can turn this into a marketing opportunity, by telling current and prospective clients about the loser who wouldn't listen to you and is now sitting on a ticking CP2000 time bomb she refuses to acknowledge.  Great example of why your expertise is needed, and DIY can be a disaster. 

Too bad the adjustment is under $5,000 so the 20% Section 6662 penalty won't kick in.