First, read the document or have your client contact the company he received the Deferred Comp 1099 form and ask why it was reported that way.  It could be a simple as the admin person doing the 1099's didn't know the rules.

Second, if they insist it is correct then you have to get the documents from your client to make sure it really is Deferred Comp and not something else.  The contract will spell out what the compensation is for.

Third, if it is truly Deferred Comp and has formerly been subject to Social Security & Medicare then here is what I would do.

Report as income and expense on Sch C so zero profit.  For the expense put the explanation "Deferred Comp previously taxed for SE tax but reported on wrong form.  It might take 2 lines but you can get it to work.  Then report in in Other Income on the 1040 not subject to SE tax.  Put in the same explanation.  That way you will be reporting the 1099 MISC for the computer matching and subjecting the income to income tax but not SE tax.   Before you do this really make sure you have documentation that it is truly Deferred Comp.  You need to see the documents yourself.

Hope this helps!