The general rule in PA is that employment is "at will" absent an express agreement to the contrary. At will employment can be terminated for any reason not amounting to discrimination on the basis of a legally protected trait (race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination is fair, reasonable or even true. So, in general, it's not against the law to terminate an employee based on conduct they engaged in years ago that does not have anything to do with their job.
In order to have a legal claim, your daughter would need to argue that she was either terminated in retaliation for reporting sexual harassment (a legally protected activity) or terminated because of her gender (a legally protected trait). If only she was terminated and the male coworker was retained, that would be very different. But since both her and her male colleague were let go it's very hard to make the argument that discrimination or retaliation were at play. The employer will claim it was simply because your daughter displayed poor judgment in allowing her picture to be ta***** *****ke that, and that her poor judgment was the reason she was let go.
This is an unusual case, though, and I don't think it's unreasonable to assume that if your daughter were male and the picture that circulated was a picture of a naked male body that she would not have been fired and this would not have become such a big deal. In that case, her termination would be discriminatory on the basis of gender. But the question is how do you prove that? I can't think of a good way, but that most likely would be your daughter's only route to legal success. She can't lose by filing a complaint with the Department of Fair Employment & Housing, which is the first step she would need to take to pursue a lawsuit. Filing a DFEH complaint is free. See here: http://www.dfeh.ca.gov/Complaints_ComplaintProcess.htm
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