I agree with PPs--"I am happy where I am"--but what about a cc: of the email and your reply to your current boss when you reply to former evil boss? That way everyone is clued in about your decision, and evil ex-boss can't twist the situation. Or is that too much?
I was going to say bcc, but cc is good so it shows bad ex-boss that current boss knows about the offer.
Everywhere I've been, that cc would actually make the OP look bad. It would look as if she had no discretion and as if she were a pot-stirrer.
Her plan to "let it slip" would be the best procedure, and actually might be expected.
(I wouldn't bother to save the email.)
That's interesting as I never would've looked at it that way. I look at it as the OP letting her boss know what's up. Where I work now (and in at least one previous place), I will cc higher-ups when I think an e-mail situation is going/may go awry (and given the history OP has with this person, it looks to me like it already is, by virtue of the fact the person, knowing what she's done to the OP, is trying to recruit the OP).
Email is "of the record," especially cc. Personnel matters are seldom emailed, especially not delicate ones like "is she poaching?"
My boss would expect me to personally tell her about the approach, but an email would be too "permanent" and too official.