Just got back into town. My advice would have been to let her know that you can't stop her, but that:
a) you appreciate her being honest with you, and telling you; and
b) considering all that's happened between the two of you, you are NOT AT ALL comfortable with this.
If she responded with "You have nothing to worry about," then just say "Please don't tell me how to feel -- considering all that's happened with you and OM, I feel threatened here, and I don't think our marriage is in a strong enough place right now for you to intentionally add yet ANOTHER person into the mix that's not your husband."
And leave it up to HER as to how she wants to respond to your concern.
Then again, she's never cared about your concerns in the past, so I don't expect her to now.
