My husbands ex has done something I really don't understand. We have been together for 11 years. When he was?

married to her she had several affairs. The first one was with a man in their neighborhood. When they were separated he slept around a bit and then the week their divorce was to be finalized she called him for a reconciliation. It has been over 20 years ago. We received a call the other night that a child was born to a woman he slept with during that period of time. We invited the child and the mother to our house. The mother came with him and stayed in a hotel. While she was at the hotel (one of his children told her where she was staying) His ex wife invited her over to her house to spend some time with her. I think this is really weird and I am freaked out by this. Why would she do this? I stay out of my ex husbands life, I respect him. Why?