Blood isn't everything Molly! They have 3 children, adopted or not, the child is still theirs.
I don't see why she shouldn't be invited. My aunt left my uncle many many years ago, should I now have nothing more to do with her as she isn't related to me by blood or, anymore, by marriage. She is and always will be my aunt.
People move on but can be perfectly adult enough to maintain good friendships over and above breakups. I got on well with my ex's ex wife. It was all very amicable, we went out for dinner together with her and her new partner, swapped Christmas and birthday cards etc... and I never had a problem with them meeting up for lunch to catch up.
Made things a lot easier for a lot of mutual friends and I never had a problem with it, never needed to. They were together for a long time and I respected that, I was never given a reason to not mind.