Boyfriend spending Christmas with his ex wife

My boyfriend has custody of his 3 kids and his ex pays him child support. This year she was hoping to see the kids for Christmas and they want to see her. My boyfriend said he will have to go for that to happen. The reason he has custody is because his ex has mental health issues. He will never bad mouth her and knows she loves her children. But her brain can switch really fast to homicidal/suicidal. When the flip starts it's usually not super strong and she can message someone and say she's feeling homicidal/suicidal but he says he would be an hour away and it would just make him uncomfortable. She has a sister who checks up on her once and while but he is worried that time she her brain switches her sister might not be able to even get there. Idk how to feel about this.

I get it being about their safety but he said she's never hurt the children and he would like to think she never would. He is just worried if her switch is too strong she MIGHT hurt the children.

Look you women KNOW you would never let a mentally ill father around his children. So why is it different that she's a women. Why not just keep his kids away from her if he's so worried.