Maybe.

Why is IRS disallowing the deductions?

How did your client act reasonably and in good faith?

They have detailed listing of the travel and meals. Business purpose not always well documented.

Abatement is not the right word, but you can challenge the assessment if when it was first proposed, the auditor did not have written supervisory approval.  

Do you think that the taxpayer acted reasonably and in good faith  is the best argument?

It sounds like that is the only one you have and from where we are sitting, we can't determine whether they really acted reasonably and in good faith.

"They have detailed listing of the travel and meals. Business purpose not always well documented."

Uh huh. You would have determined there is evidence that it met the qualifications.

"Do you think that the taxpayer acted reasonably and in good faith is the best argument?"

Did your taxpayer know about, "I have always in the past gotten this abated?" As in, you submit, wait to see if it flies, or has to be explained afterwards?

You stated this audit is 14 months along; given the timeframe and delay, you know the requirements for that tax year in question. Only you can determine if you filed what you filed with reasonable due diligence.