You believe so or you have evidence to prove so?  Especially since the couple already separated and may disavow that the marriage ever existed, you may have an impossible task to prove it even if they were indeed in a common law marriage.  Did they file any declaration with the county clerk and did they apply community property laws before and when they divorced?

Agree with Bob that they presenting their story to the IRS, whether or not those are facts, is very different from you representing them before the IRS.  You already know the return is fraudulent (or at least has expenses that lack contemporaneous records), what documents are available to back up the corrected return now and how confident are you that those records are genuine, complete, and accurate, not only for expenses but also income?

With another 10/11 days to go before the corrected return must be filed, are you sure you are able to prepare a complete and accurate 1040X on time (without all the facts and documents already in front of you) and fairly represent the couple, from a due diligence and professional practice standpoint?

As you are aware, the issuance of 3167C letter is an administrative concession of sort.  By caselaw, fraudulent filing cannot be undone by the filing of an amended return.