Innocent Spouse
Your mother may not be liable for this at all. I don’t know if the payroll tax liability would come under the following provisions, but I think it might.
Relief from joint liability. In some cases, one spouse may be relieved of joint liability for tax, interest, and penalties on a joint return for items of the other spouse that were incorrectly reported on the joint return. You can ask for relief no matter how small the liability. There are three types of relief available:
1)Innocent spouse relief, which applies to all joint filers.
2)Separation of liability, which applies to joint filers who are divorced, widowed, legally separated, or have not lived together for the past 12 months.
3)Equitable relief, which applies to all joint filers who do not qualify for innocent spouse relief or separation of liability and to married couples filing separate returns in community property states.
Publication 971, Innocent Spouse Relief, explains these kinds of relief and who may qualify for them. Click on the following to download a copy of Pub 971:
http://www.irs.ustreas.gov/forms_pubs/index.html
I would suggest you get Pub 971 and see if you think it applies to your mother.
But in any case, I strongly advise you to hire a CPA to handle this for you. A good CPA will be able to tell you if your mother is liable, and if she is, to work out a satisfactory settlement.