The box 13 on the form W2 indicates that your husband was an "active participant" (for any part of the year) in the employer's provided retirement plan.
Generally, an employee is an active participant if covered by
(a) a defined benefit plan for any tax year that he or she is eligible to participate in or (b) a defined contribution plan (for example, a section 401(k) plan) for any tax year that employer or employee contributions (or forfeitures) are added to his or her account.
So far - your employer should check this box - as long as you are eligible to participate in a defined benefit plan or employer's contributions are added to a defined contribution plan.
As long as neither your husband nor his employer added any contributions to the 401k account - your husband is not considered an active participant and the box 13 should not be checked.
You may want to discuss the issue with the employer and ask to amend W-2 form.
Please see for reference instructions for the form W-2 - http://www.irs.gov/pub/irs-pdf/iw2w3.pdf for Box 13-Checkboxes
Let me know if you need any help.