I would ask nicely to speak to this auditor's supervisor. This person needs to be able to answer the question: "why do you believe this is backdated?" to someone like their boss before you agree to anything.
I will admit I worked for the IRS in the '80s on the income tax side but we were told we had to agree to a meeting with a supervisor and the taxpayer if it was requested. Assuming that is true that strikes me as a good first step.
I haven't worked for the IRS since the '80s and it was on the income tax side. But we were told all the time you had to always agree to allow a meeting with the taxpayer, your supervisor and you if asked.
I would start with a friendly request to have a meeting with the auditor and their supervisor to make sure everyone is on the same page in regards to the issues at hand.
This will get you a fresh set of eyes and most supervisors have many years of experience in the field and were willing to reign in an auditor that was way off base.
Generally, a "tax dependent," either a qualifying child or a qualifying relative,
http://apps.irs.gov/app/understandingTaxes/hows/tax_tutorials/mod04/tt_mod04_01.jsp
is eligible for coverage. As Chaz says, check the plan/SPD.
In addition, Michelle's Law may extend coverage eligibility for up to a year past age 25.
http://www.dol.gov/elaws/ebsa/health/employer/657.asp
The plan document may provide for coverage for an adult child who is disabled and who meets the requirements of being a dependent of the employee. The only way to know is to check the plan/SPD.