It doesn't mean she will win the lawsuit. Anybody can file a lawsuit for any reason.
As a general rule in an at-will state, an employer may fire an employee for good cause, bad cause, or no cause.
There are exceptions. In addition to state exceptions, a state at-will employment law does not override federal employment protections contained in such federal laws as the Civil Rights Act of 1964, the Americans with Disabilities Act of 1990, the Age Discrimination in Employment Act of 1967, and similar federal laws.
I don't know of a federal employment protection for voluntarily appearing on a soft-core porn Internet site. Ms. Black wasn't a member of a protected class.
But don't trust what I say. I never practiced employment law. Please research it yourself or wait for a FReeper who is experienced in employment law to comment.