Wow both your ex-gf and husband were dicks about the situation.
Your girlfriend for doing it in such an impersonal and cowardly way. But atleast she got it out and is not stringing you along anymore and give you mixed messages. It is always hard to break up with somone you actually still like but just don't feel romantic about anymore. And if the only way she could get the courage to break up was this way then it is better to get the break up message, than to wonder whats going one for more months in the future.
Also I doub't the friendship from her side can be that true or strong if she agreed to let your husband stay with her after he knew you had just broken up with her. A good friend/metamour would have kicked your husband out and rescheduled their date night for a next time, because you needed him more, and letting him stay was very bad form, after she knew you were alone hurting and your other partner was away from home/with her.
Your husband though was a real insentsitive dick for staying with her that night when his wife actually needed him for emoional support and comfort. Date night or no date night. Plus he basically stayed with the person who just broke up with his wife! So it was like adding salt to the wound of nyah nyah our relationship is still going strong, sorry for you hurting wifey but the two of us now need to celebrate the gf new found freedom! WTF!?!?! Was it the NRE? But that is still no excuse.
What is he going to do when she has a crisis on your and his date night? Tell her sorry I'm on a date! Or if you have a more serious crisis in their another date night?
OP I would caution you to not be too accomodating in the near future in your interactions with her. Don't try to be a super woman who is her bestest friend because the two of you agreed to stay friends after the break up. Make sure that you will take the needed time and distance from your ex-gf so that you could emotionally move on from her, before resuming things with her in the friendship level.
For her it will be very easy to categorize you as a friend and resume things like that, because she was the one who has moved on emotionally a long time ago, and it will not hurt her to have you as her friend, plus also probably help her deal with any guilt issues she might have about stringing you along, because then she can say to herself "See we are friends now, both of us are fine!"
If you don't let enough time and distance between your break up and resumed friendship there is a real danger that you will not move on easily, and she will unintentionally string you along again. It is hard to regatecorize someone in to the friend zone if you see them still in a romantic light, or were with them a week ago and now wham you are suddenly only friends.
It will be very hard to feel the new line between a friendship and romantic relationship if there is not enough distance for you to see what is normal between friends because you will be too used to interacting with her in a girlfriend level.
So don't pressure yourself to handle things better and surpress your pain, if you notice it is harder to just be friends than you thought. And don't let your husband or ex-gf do it either.