I don't have children. However, I do know that my parents would have never dreamt of accessing my private anything after I became an adult. We were taught, from the time we were teens, that we would be our own persons upon legal adulthood. They prepared us for it, both the rights and responsibilities. Because we were taught this, our parents would have faced our wrath had they invaded (which they wouldn't have, but just 'if') our privacy.
Given this upbringing, this invasion of privacy is inconceivable to me. There is no excuse for the behavior of the woman in the OP. Quite frankly, if I were good friends with a family in this situation, I would risk my friendship and inform the other adults whose privacy is being violated. To me it is equivalent, or at least nearly so, as anyone else invading privacy in such a manner. In all other cases, I would inform, so I would do so here. I feel it is that clear cut, and that strongly about it, though I do understand anyone who feels differently on that score.
The interesting point to be made about informing is this: If the young adults don't care, then no harm done and the friendship should survive. But if they do care, then all the more they should be informed.