In the united-states, those actions would be fully protected by the First Amendment. Andy has a constitutional right to speak freely about essentially whatever he wants -- including Mary's criminal conduct -- unless his speech falls into one of several narrowly defined categories, none of which would apply in this case. And because Mary has already put this information out on the Internet, it is likely not sufficiently private to support an invasion-of-privacy claim.
However, the unfortunate reality is that complainants, police, prosecutors, and judges frequently ignore First Amendment protections. Indeed, many states have laws against "cyberstalking" and "telecommunications harassment" that are incredibly broad, and that clearly apply to conduct protected by the First Amendment. For instance, Ohio's telecommunications harassment statute makes it a crime to send an e-mail "with purpose to abuse, threaten, or harass another person."
This means that many people who engage in First Amendment-protected speech end up getting prosecuted anyway. If they pay for a good lawyer who knows how to properly raise a First Amendment defense, they may escape any penalties. But because most defendants do not have those resources, and because many lawyers are unaware of the First Amendment implications of such prosecutions, most defendants in such situations likely end up being convicted despite behaving perfectly legally.
In the united-kingdom, though, the situation is very different. Even if Mary is breaking the law, and even if Andy limits himself to strictly factual information about what's he's learned about her conduct, he may still be held civilly liable. Mary may also be able to pursue Andy criminally for harassment if his e-mail causes her substantial emotional distress, and civilly for "harassment by publication." The fact that this information is already public is likely not going to go very far in changing the analysis.
Andy's best course of action is therefore to play it safe by keeping his mouth shut. Doing so has both legal and nonlegal benefits: He avoids exposure to the hassles of defending himself from criminal charges and he gets some time to examine the resentment, jealousy, paranoia that makes him want to humiliate his "friend."