Your perspective was very helpful, nycindie, as was rereading the correspondence, and considering what I know about my friend. I have decided that she is not very self-aware and did not consciously mean to hit on my boyfriend, although it certainly looked that way. I think my perspective was overly informed by what I know about my friend and my husband. She has offered him sexual play repeatedly over the course of their two-year friendship, and he just keeps declining and saying he sees her as just a platonic friend. She only stopped recently, after she told me that she had decided that my husband is emotionally poly, but sexually monogamous. Well, she can think what she likes to spare her ego, but my husband has a new girlfriend and they are very likely to be sexual in the near future. (Hopefully that won't encourage my friend to start chasing him again.)
So, I think I was already in a place of irritation over what had been going on with my friend and my husband, since I do find it creepy when people keep trotting out their sexual agenda after you've said a firm "no," and thus was predisposed to think that she was just moving on to another of my partners. And it did LOOK that way. But I have told her that I don't believe she was doing that, and I think we are going to be able to stay friends. At least she knows now that I would expect her to discuss things with me if she fancies yet another one of my partners down the road.
Thanks to everyone who took the time to offer their insights.