The only person your husband can control is HIMSELF. It may be hard at this point if he's been engaging for a while to SUDDENLY quit....but if he doesn't want to act tat way, he doesn't want to.
If he feels uncomfortable being the grumpy bad sport, he can lightheartedly roll his eyes -or even show exasperation that they're so lame-when THEY do it while NEVER engaging. Maybe they'll get sick of his lack of participation. If this girl wants to provoke your husband so bad, he could blow her mind one day with a little wisdom. "Hey, Shelby Sue Fluffypants, all joking aside, you should watch your reputation for the sake of future jobs. You could be "unfairly" judged for "falling victim" to the "unprofessional environment" here. I admit I was doing it too, but it's getting old." Or something. That way he's not bad mouthing the boss, OR blaming her...
Or if he's more comfortable joking around with them, he should make his role the clown who encourages THEM. Like when she tries to flirt with him, he could say, "Uh oh, boss, your woman is out fishing again and I don't want her" or something. Just shine the light on their glorious in-office "relationship" and she she says anything like, "But it's you I'm after now" Or whatever--what could this girl possibly be saying ? Then your husband can make sarcastic little, "Yeah...NO." comments always shutting her down and NEVER flirting. Girls GET IT when you NEVER flirt with them. I've had countless jobs and I was a cute young thing. There were the men who tried to flirt (who I did not indulge, duh.) and the men WHO DID NOT TRY TO FLIRT. Your husband wants to be one of the nice, respectful men who DOES NOT FLIRT. No matter how she acts. If she really shoots him a zinger, he can blush and walk away, but NEVER RESPOND OR ENGAGE.
To be honest, if my husband had one of these ladies at his job, I'd be in her face. I'd invite her to lunch all friendly style. Just the invitation should scare her away. Or walk in and say "Oh YOUR Tiffy Buglebutt, I've sure HEARD about you. Everyone has." All civil but unimpressed. Or just straight up approach her and say, "Oh, just so you know, some people have been talking about your behavior here. I'm sure you're not as bad as they say, but for whatever reason, it's bad, so don't let "other people" tarnish your reputation." Or something. Annoying.