You make some good points, AnnabelMore. My former metamour did indeed hide what she was very well, for a long time. As I think I mentioned, I had thought of her as one of my best friends, and so did my husband. We were both poor judges of character, by your definition. My ex is still with her almost two years later, so presumably he doesn't have any problems with her character.
The main reason I don't want to hear about my boyfriend's new friend is because I know it's just going to upset me, without accomplishing anything. He won't not date her to avoid upsetting me. We've already established that. I can't stop a relationship from happening if they both want it, and if it does happen, I have zero faith in my ability to stop anything bad from happening to him or us, from 2000 miles away. Sure, I could talk to her. If she's a good person, asking her to treat him well and respect my relationship with him is not really necessary. If she isn't, I'm just wasting my breath.
I got mad at him because he agreed to a boundary (not discussing her with me), which he then "forgot." I really don't think it should be a big deal to ask him to not discuss one particular thing with me, considering how I feel about it. It's not like I live with him and he's constantly having to leave out details of how he spent his day. We don't even communicate every day, and when we do, it's usually via instant messaging or email, where it should be very easy to censor yourself.