N.S.
She probably did it by accident. Let her know, I think you may be legally obliged to do so. E-mail is private (especially if this is a public school which would then be considered a public institution) and there may be legal ramifications as well as ethical if certain information gets sent out such as budget information. That sort of private and sensitive information in the hands of someone like an investigative or education reporter or disgruntled parent could be a problem for her if it gets back to her bosses. Furthermore, what if she accidentally releases personell information, or student records in the future. You probably aren't the only non-school person who got that e-mail, since she probably did a general e-mail from her contact list. That's definitely something she needs to know.
This made me think of a recent story in the national news, where a jealous TV anchor had been somehow getting access to his co-anchor's private e-mails, and used the information he learned from them to sabotage her career. She was fired based on some inside information her co-anchor found through snooping in her e-mail account.
Somehow, long after she no longer worked for the station, she found out what he was up to, and filed a bigtime lawsuit against the still employed anchor and the t.v. station. Apparently, it wasn't enough to sidetrack the woman's career, he kept up with the snooping and sabataging long after she left and kept leaking gossip to the other local media. I think that's how he was eventually found out.
Anyway, the female anchor was finally awarded a settlement, and needless to say, the t.v. station lost millions, the male anchor's career is now finished and he's in disgrace. There may even been some federal charges because tampering with e-mail is a federal crime. I know an unrelated situation, but it's an e-mail in the wrong hands case just the same. Just food for thought.