Because to this point, the husband hasn't said anything to her about it, as in.... asking her to stop. (Unless I missed a post?) So as far as this lady knows, he either doesn't care, or enjoys it.
You have a point here - that the DH should say something - but that does not make her behavior anywhere near approaching appropriate. In my view, it is not okay to pursue someone you know is in a relationship. I grasp some people have various styles of relationships, but unless and until someone who is in that relationship invites you to join, pursuing something with someone in a relationship is always rude as I see it.
Exactly. It is rude and disrespectful to both the person being approached AND their SO. If you don't know they're already spoken for, that's one thing, but once you DO know, you back off. Now if your intended target actively encourages you, that's one thing, and then you're both being rude and disrespectful to the SO. But that's another can of worms for another thread.
The catcalling and whistling is sexual harassment and nobody
should be on the receiving end of that. One can hope that Tammy is just clueless enough to think she's being harmless or flattering, but he needs to tell her to stop (she seems likely to register pretty low on the clue meter, so I think polite but blunt is likely the way to go), and if she doesn't, she should be handled in the same manner as one would recommend a female handle a male creeper - cut direct, document, and a chat with building management (&/or Tammy's landlord, since (IIRC) the OP & hubby own their unit, but Tammy rents hers).
Hopefully a polite, direct request to stop will be the clue-by-four Tammy needs to snap out of it, but OP & hubby should have a few next moves in mind just in case it isn't.