It pretty much is. Especially if she isn't okay with it. I mean, if she had moved on and was supportive, that would be one thing, but... if she hasn't and you know it? Definitely bad form.
Maybe you could. Even if it doesn't got as anticipated, maybe closure will still be had and that will help, regardless. Do you care for Guy B, and not just because of the history? Because history also makes it difficult when it ended badly before.