Awesome Twinkie, just awesome.
When it comes to protecting your home front, NOTHING is out of bounds. Do not let social etiquette / morays stop you from publicly humiliating anyone who threatens your family. Public, public, PUBLIC. All sense of "privacy" of the issue must be completely eliminated by you.
Do not let fear of embarrassment (to yourself or your husband) stop you. Talk about it, OUT LOUD, to everyone involved. And when you do, you're not being crazy, you're confidently acting as if what YOU want to occur is already a foregone conclusion. Confidence!
Walk into his work, take him by the hand and walk him over to her desk... Introduce yourself confidently and unashamedly. Tell her you've been concerned lately there may be something inappropriate going on (or starting to) and you wanted them both to know that:
A) "I love my husband, so it's not OK."
B) "If anything does happen, well, oh my, no telling WHAT you'll have to do make your life impossible to live."
C) "Not you honey, don't worry, she's the only one you'll be "impacting"... And we'll just laugh about it afterwards, won't we?"
D) Kiss-kiss...."Have a good day...oh, do I need to talk to your boss, too? No, I can do that later...ta ta."
Picture those strong female hollywood types....BE that person for a few minutes and enjoy it.
Then make sure your husband knows you're having fun with it, but you're not unaware. Then have a great dinner.