Ex-husband's new girlfriend is scaring our kids. He is vying for custody. What can I do?

My ex-husband and I both work in the hospice industry. That is how we initially met, actually. We have a 13 year old and a 9 year old. My husband and I divorced 3 years ago this month and he has been a handful ever since. Lately, he has been acting very, very strange. I am not sure if this is just to get under my skin or what but he has begun dating a woman who was once in his care at our old firm. The woman in question is a severely low-functioning down-syndrome patient. When I say severely low-functioning, I mean it. She is not capable of any choice cognitive functions or basic personal-care functions. I have no idea what has gotten into this man. He has our kids every other weekend. They say she has been screaming, flailing about, knocking things off the table and lunging her arms towards them attempting to tug on their hair. I am at a loss for what to do. He is supposedly engaged to this woman with her family's consent. Since he doesn't work for the same firm that takes care of her he is not in any conflict on those grounds. He said he is looking foward to starting a new family with this woman and said once the marriage is finalized he will petition to gain more frequent custody of our children. What do I do about this!?