If it was me, I would also like to know... but most likely, if they get a random person telling them that, they will consider it fake or after addressing it with the husband, they are most likely gonna believe them more than a random anonymous person on facebook. And then, husband will just move his encounters with the 19y/o somewhere else more secret where they won't be found again.
At least that's how it has gone on my experiences telling girlfriends when their boyfriends are cheating.
People are always more likely to believe their cheating partner than a random person without proof.
I would do what you say, but don't be surprised if she doesn't believe you or if she decides to do nothing... still, for your peace of mind, if you want you can always try