I am going to ignore the posts about STIs and judging risk and whatnot. The OP has repeatedly requested that we focus on her actual issue, not the side circumstance of how to handle STI risks. I don't find it useful to keep harping on that.
OP, it seems to me that your main concern is that you don't - perhaps can't - trust your partner's judgment. I was unsure from your posts if this is a pattern for him and you. You see a metamour behaving badly, perhaps trying to damage your relationship and he does not see that at all. Is that accurate?
Ask yourself if your partner is fundamentally trustworthy. If he is widely untrustworthy, you have bigger problems than his inability to see ill intent in a metamour. If he has acted as a trusted partner to you in all other ways, does he have this one odd blind spot of being unable to believe or even see when someone is destructive?
Also ask yourself if your perceptions are the entirety of the situation. It is totally possible for you to see her as a conniving bitch and for him to see her as a misguided friend. Both may be 'true'. His perceptions may not be 'wrong'. Yours may not be always 'right'. I'm not saying you are crazy or inaccurate in what this particular woman was doing. I'm convinced. But it is really helpful to get out of your head and think about other ways of seeing the same thing.
It is unlikely that he will ever perceive the same people the same way you do. Especially if this is a fundamental part of his personality. Someone who is generous and always give the benefit of a doubt is unlikely to become someone more suspicious. And as others have pointed out, you don't want to change this behavior completely.
And you also do not want to get into the winless game of who is right. Being right and him being 'wrong' goes either of you no good. Telling him he is naive, misguided and such - even if this feels totally true to you - will not help. Trying to change him is an strategy unlikely to work long term. (If he wants to develop better bullshit detectors and does so on his own, that's a different thing.) You and he will likely NEVER be on the same wavelength in perceptions of potential or current metamours.
But what you can do is agree to make each other feel safe and respected. He can acknowledge your concerns about another's possible ill intent and bad behavior. He can not dismiss that out of hand. But he does not have to agree with you. You can acknowledge that he sees people very differently than you do and that way is not necessary wrong or incorrect. But you do not have to agree with him either. You two can then work together on ways to make each other feel reasonably secure and safe, while being respected and heard. (I say reasonably because perfect safety and security is not possible on this plane of existence. Even the most loving and trustworthy of spouses may get hit by a bus.) Galagirl had a bunch of concrete suggestions on this.
Disagreeing with you should not make you feel unsafe. Not seeing the same things you do should not make you feel unsafe. Actions or lack of action cause insecurity and lack of safety. However, he might be doing something that contributes to your feelings of not being safe. Beyond not seeing the same things you do, what is he doing? This is the place to check your perceptions and feelings again. Just because he is talking to someone untrustworthy does that really impact your relationship? I understand your fears of why this might be true. But feelings are poor indicators of fact. He can talk to that conniving bitch and never have the slightest need to leave you or change your relationship in some way. The same problem of him not seeing the same issues with her that you do may actually help insulate him. He does not see the conniving activities and words and so does not respond to it. He's just talking with a friend in need who is having a hard time! Sometimes thinking the best of someone, and maybe some naivete, is its own protection.
Perhaps there are steps both of you can take to 1) respect and consider the other's perceptions and viewpoints, and 2) reassure each other. If he still talked to this person, and you saw that it had no effect on your relationship, that you were not thrilled but realized that your feelings about this person were not the same as the reality of your healthy relationship with him, then your perception of his inability to see what you see may shift. It would become a thing about your partner - like his inability to dust that one spot or your inability to understand English Premier League soccer (to use stereotypical examples!) - that you take in to account, accept and deal with as life goes on.
It's hard when a loved one appears to live in a totally different universe than the one we inhabit. It can make one question one's own validity. I struggled with this somewhat in my marriage. My ex and I were very different people and sometimes did not see things in a similar way at all. It's a special kind of maddening. But the very thing that makes them so different from you is often at the heart of why you love them. Try not to lose sight of that while you and he work through this.