What 'tax liability" are you referring too? I'm going to guess her options are under the 'incentive stock option" rule because of her position so there should be no tax considerations until she trades them. On the other hand if she did sell them instead of holding them she would have been taxed at ordinary rates for the gain using a base of $8.70. The $8.70 would be deducted from the sale.
Her only obligation would have been to pay the $8.70 per share at this point.