The problem is that it doesn't matter how much she ignores this girl, her husband isn't ignoring her. Sounds like he doesn't want to block her, but OP hasn't directly said that she asked, only that they talked about it.
Essy you should call her out in front of everyone. Next time you're all at a gathering and she won't stop blabbering to him, ask her in front of everyone if she has a crush on hiim. Don't yell, or seem upset, just be matter of fact and point out how much she in contact and how she focuses solely on him within the group, an then watch her turn white.