I don't see the connection with the First Amendment protecting the right to photograph an unsuspecting person's private area in a public place. The better argument is that there must be a specific statute on point criminalizing the statute. Such a statute, to be constitutional, must be tailored in a limiting and specific way to achieve the narrow purpose of the law.

If, as the defense claims, the defendant merely took a picture of the woman, who was inadvertently revealing a view up her skirt in a public setting where the defendant had a legal right to be, and the photo memorialized the vantage point he legally had, there is no crime. If the defendant had to lower or position his camera angle to create a view that he would not reasonably had by his mere presence, then the woman's expectation of privacy is heightened and he could be subject to prosecution if there is a specific statute outlawing this.

I don't know what the MA law is, but if he is prosecuted under a peeping tom law, the case may not hold water as those laws prevent voyeurs from invading privacy at home or a place where there is a legitimate expectation of privacy.