so if she got a test now and it was negative, would it alleviate your concerns? Because then, everyone your boyfriend sleeps with would be "clean" right? You are perfectly happy to assume your husband is clean based on the test results of other people so her having a "clean" test, the person that your partner is actually sleeping with, should be more than adequate for you. It should be more reassuring than the negative result of someone that neither you or your partner are sleeping with.
You're also ignoring this: there is a theoretical risk that although your metamour tests negative, her new partner (who is untested) could have something like herpes and just hasn't given it to her yet. This is also true for your untested husband who could have something like herpes and still, at this time, have partners who haven't contracted it yet, especially since he uses condoms with you and the other people he is having sex with. But so does your boyfriend, right? He is only fluid bonded with her, not you, so if his wife did get an std that he contracted, you still have the protection of condoms. You have oral sex with your husband, I assume, without condoms and you have no idea if he has an std - you just assume he doesn't because nobody he sleeps with has caught one. So you are assessing the risk differently seemingly because one person is your husband, and the others are our boyfriend and his girlfriend. I'm not on about this so much because I think herpes is such a huge risk or a terrible thing to have. It is just painfully obvious to me that you have different rules for different people that are based on things other than science and logic and you have a real issue dealing with your partners and metamours dating people who aren't known to you.
In my opinion, if this wasn't about the way you prioritise and privilege relationships, you would be perfectly happy to continue as you have been if either your boyfriend and/or his wife produce negative test results. There is no logical reason for you needing any test results from this man, especially when you don't need them from your husband.