Concerned About Custody And Protection For My Friend - What Are Her Options?

I'm looking for advice regarding a woman I know (I'll refer to her as Sara) who is in a long-term relationship with a man who I will refer to as Dave. They’ve been together for 8 years, and Sara has two children with Dave (a toddler and a 7-month-old baby). Dave also has two teenage children from a previous relationship, who live primarily with him.

Sara has been the primary caregiver for both of Dave’s children and their own kids, emotionally and physically, for most of their relationship. Dave’s involvement with the children is mostly limited to financial support, and he’s been largely neglectful in providing basic care, like feeding, diaper changes, and attending appointments. There have been multiple instances where he’s been verbally aggressive toward Sara, and today, I witnessed him being threatening enough that I felt compelled to secretly record audio out of fear he might become physically violent.

The argument was as follows: While Sara was working from home, their baby was crying, and Dave (who was playing a video game) "fed" the baby her bottle. After this, he tried to get her to sleep, but left her crying on the bed, claiming she would "fall asleep eventually." After this argument, he left to hang out with friends. When Sara later checked, it was clear that the baby had been upset because the bottle was only half-empty—Dave hadn't finished feeding her. He got frustrated with her crying and, instead of finishing the feeding, tried to put her to sleep, but this didn't work. When his attempt failed and he became embarrassed in front of his friends, he shifted the blame onto Sara, even though she was trying to work.

After he left, I spoke to Sara regarding why she has not left him when he obviously treats her and the kids horribly. Sara is scared to leave Dave because she believes he would take her to court for custody. She’s worried he might try to gain partial or even full custody of their kids, despite his neglect and lack of responsibility for them. She doesn't trust him to be alone with the children, even for short periods of time.

What options does Sara have to prevent Dave from obtaining any custody, especially without supervision? I suggested she document instances of neglect, but I’m concerned this might not be enough or could even backfire. Could she be accused of neglect if she tries to collect evidence of his behavior? I’m stuck, and I just want to help her find a way to protect her children.