An employer is generally not covered by HIPPA. HIPPA only applies to medical providers and providers of health care plans. I am not saying there isn't some violation of privacy laws - I don't know, but I doubt its HIPPA.
And if the real reason they fired her was for having premarital sex then there is no violation of federal discrimination laws unless there is proof that the employer knew about men having premarital sex and did not fire them. But there could be a retaliation claim for requesting FMLA leave.