The IRS goes by box 13 Retirement plan.
See Zbucklyo's answer. If the employer provides a defined benefit plan in addition to the 401(k), this could be the reason that box 13 retirement plan is marked (even if your wife does not meet the vesting requirements).
If the box 13 marking is truly incorrect and the employer refuses to correct it, the only option is to submit a substitute Form W-2 (Form 4852).
