This is for the OP.
Look, we get that you're scared. We get that you want her to be the one at fault because your husband is the breadwinner and if he had to take responsibility for all this, then he loses his job and you don't have any money. We understand that you want him to be blameless because if she's the one at fault, your life goes on without any new difficulties to overcome while your expletive of a husband continues to bring home the bacon. We get all that.
The fact remains that legally, she is not the one at fault here. Legally, your husband is the one in the wrong. Yes, it would have been better if she hadn't had an affair with a married man. But by your own accounts, she didn't know he was married at the time. HE DID. He is the one who broke his marriage vows. He knew he was married right from day one and he still went ahead and had sex with her. He is the one who broke police policy. She didn't make any promises to you. She isn't responsible for following police policy. She didn't know, initially, that he was married. That's all on your husband. No matter how much you want it to be her fault, it wasn't. It was his. And even if you live in one of the states Red mentioned, your case against her is weak at best. If you live in any other state, your case against her is non-existent.
The one you have legal recourse against is your husband. It's called divorce.