I strongly disagree with some of the previous posters and stand by my suggestion to ring her up and ask her, "what's so important that you have to contact my DH every day when you know he's a married, family man?"
This kills all the birds with one stone.
If your DH kicks off, you know you've hit a big nerve.
If you ask your DH about it before ringing the OW, you know you won't get the truth.
She will either back off, flummoxing, or will apologise and be more careful about the boundaries of a platonic friendship with a married man.
Either way, you have nothing to lose.
Taking the bull by the horns shows that you are not a woman to be messed with, and your DH, if he has anything about himself, will be proud of you for it.
I challenged a woman like this many years ago. My DH was very much "thank fuck for that" because he's a nice friendly guy who didn't quite know how to tell her to piss off without coming across as aggressive.