There are two halachic concepts -- hair as erva, and a married woman's obligation to cover. If hair is erva, why is uncovering okay before a woman gets married? Numerous poskim (Ben Ish Chai, Aruch HaShulchan, Rav Moshe Feinstein) all suggested that this definition of erva is subjective, and no one is shocked by a single woman with uncovered hair.
If so, why do poskim prefer (though there's leeway in many situations) that a widow continue to cover her hair? Either that's simply the norm that evolved, or once the obligation to cover kicked in, it remains in effect (or alternatively, the obligation to cover turns it into a sort of erva) -- there are mp3s on yutorah where Rabbis Broyde and Willig address this issue.
I've never heard any contemporary posek suggest that non-covering had anything to do with virginity -- and actually, I find that quite offensive.
Many divorcees and widows have heterim to uncover their hair, for job or shidduch reasons. This would certainly apply here. But putting that aside -- if it could be argued that societal norms treat a "mom's" hair as erva, then maybe you could argue that she should cover. I can't think of any other reason why she should cover, though. (Now if all her friends at parent teacher conferences, including the divorced ones, cover their hair, it might be helpful for her to do so too as far as school standards or not sticking out. But I can't see any halachic argument, other than erva vis-a-vis norms.)
