You can fire her if she won't work with him UNLESS he has in fact been committing sexual harassment againt her. (Sexual harassment at work is illegal.) If he has been harassing her, you (as the employer) have an obligation to investigate and, if there is any credibility to her story, to at least take some remedial or protective action (e.g. moving one employee to a different shift; putting on video cameras or security to protect her; possibly firing him; etc.).However, if he has not in fact done anything inappropriate or improper, she--as the employee--has no right to decide who she will and will not work with; if she won't work with the person her employer tells her to work with, she can be fired, or anything short of fired: e.g. she could be suspended, demoted, have her hours or pay cut, etc. Her recourse, if she truly does not want to work with someone and  she's not the victim of illegal sexual harassment, is to seek a different job.