"0" allowances means that they should have withheld the max amount of withholding. I would recommend taking a look at her pay stubs - was there withholding on them? If so, getting the company to correct the W-2 is going to be the easiest thing to do in the long-run.
However, if they made a mistake and just didn't withhold anything, that is unfortunate but she is still liable, in regards to box 2. Withholding is only her money that they forward to the IRS, from her paycheck, as an estimate for tax.
However, this is not true in regards to boxes 4 & 6; social security and medicare. They are liable to split this with her. This is odd and hopefully it's all a honest mistake. I'd take a look at her paystub. Typically near the end of the year, there is a total of year to date withholding. Let me know what you find and I can assist you further.
