You might have her use the calculator found [here][1].

The situation you describe suggests that her ex-husband was shouldering the majority of the family tax burden.  Or that she was earning a much higher amount than he was.  As an example a household income of 20K/100K split in income increases the tax burden for the higher earning spouse increases 3K.

There is really not much hidden or unknown for the things you are speaking of and generally speaking many tax issues are pretty strait forward.  Once you start diving into business deductions and the like things might start to get hairy, but that is not the case with the situation you outlined.

  [1]: http://www.taxpolicycenter.org/interactive-tools/marriage-bonus-and-penalty-tax-calculator