Well, it is hard to determine how extensively your husband lied to her, without actually having been there. Did he indeed inform her that you were pregnant? and if so, how do we know this? because he said so? or has she said as much also?
Also consider that NRE exerts a strong force over the mind. It makes one more susceptible to the power of suggestion ... even mistruth, even if it doesn't add up ... from the partner one is newly in love with.
Has she made a formal agreement with you not to talk about him during her time with you? If not, it might be something to consider to get that kind of agreement from her, if she will give it.
Obviously, there is a long way to go before you and she can have an amicable relationship. Maybe you will have to separate yourselves from one another for awhile, linked only by your husband. But the problem there is, he doesn't seem to be innocent in all of this either. In fact I wonder if he doesn't have far to go in the honesty department.
"He wants to have this rule we leave the past in the past and not bring it up but obviously that is easier said than done ... memories, bad feelings, being hurt don't just go away just because they are getting brushed under the carpet."
See, to me, that sounds like he just wants to avoid the accountability for his own actions.