My new best friend TaxBert says this:

Per diem payments reported in box 12 of Form W-2 with code L are considered part of a nonaccountable plan and must be included in wages in box 1. These amounts are subject to income tax withholding, Social Security, and Medicare taxes.

If the per diem payments exceed federal per diem rates, the excess amount must be included in box 1 as wages. The portion equal to or less than federal per diem rates can be excluded from box 1 if proper substantiation requirements are met under an accountable plan.

DOES your client, or his/her W-2 fit into any of the above?  

And it sounds as if the adjustment for 'less than per diem' would already be done on the W-2 by the employer.  Perhaps a paystub, or a call to the employer, would shed some light on it.

But I see nothing that says 'adjust out' on the 1040.