HMR is a terrible thing for the children of the halachic “first” marriage. I had a student whose father refused to give his wife a Get but got 100 naive and well meaning Rabbis to sign a petach and he remarried. The child, and the siblings, were humiliated & devastated. It was bad enough their parents split, but this??? Mom came into school crying, begging us not to let her ex in. We agreed, because she had legal custody, but I suspect the Hanhala would have agreed anyway. My student went from being outgoing and generally secure and happy to a shell. She just shriveled like an old etrog. Things didn’t go so well for her after she left my class.
It didn’t matter the entire community, including the influential Rabbonim, were horrified and on the side of the mother. I don’t know what the eventual outcome was, whether he ever gave her the Get, but I can never forget that sweet little girl and the pain she endured. Think before you sign such a paper… get every scrap of information before you do.