Regardless of what your previous relationship was like with your GD (whether it was good or bad!), what she has said to you is unforgivable! It's something that you will never be able to forget, even if she feels guilty and comes grovelling to you with an apology. She's in the wrong and she probably knows it, deep down, and must feel ashamed about what she said.
Listen to your husband. As upsetting as this must be for you, you must try to put it behind you, otherwise, you're going to make yourself feel sick with the worry of it all. I don't know what your past relationship has been like with your GD, or your ex DiL ... were there some tensions between you and your GD's mum perhaps ...? Something has obviously triggered this reaction, but that is absolutely no excuse from a 34 year old adult!
I wouldn't try to contact her again about this, let it lie. Continue to send birthday cards, etc., if you usually do, but if she returns them, or tells you not to, then simply stop, without responding to her.
You can't do anymore than that. She's the one who created this upset. It's up to her to try to put it right, if she ever chooses to do so. If not, you'll have to learn to live with it, and know that it wasn't your fault. Put your energy and love into other family and friends who deserve it.