Won't get help

Unfortunately, that is not possible. If you want her to get treatment and she is unwilling, calling the police when she is threatening your family may be the only thing you can do. Talk to the police ahead of time. Many police dept. have dealt with this situation before. They may be able to give you some advice. You mention she has 4 children. These children deserve to have a mother that is stable and healthy. She needs treatment. Only a doctor can provide that. If she is unwilling then perhaps having her forced to see a psychiatrist is her salvation from herself.

You CAN'T get someone to think or talk or act rationally if they are unwilling to seek treatment and she has the right not to be treated. You can't force treatment anymore than you can force a heart patient to follow their diet and take their meds.

Possible resources:
Local mental health center. Tell them you need help with a family friend who refuses treatment and how to deal with them.
Someone with mental illness: They could answer lots of questions.
Detective from your friend's case: He'd benefit by telling him the story in a nonemotional way. They appreciate he-said/she-said and are usually pretty good at sorting out who's who. She must be a pretty good actress to get the cops on her side. They're usually not so quick to throw exes into jail on the word of the other party.

A 72-hour hold is available if she's a danger to herself or others. You'd have to call the cops when she starts making threats and give them a reason why you thought she's a danger. i.e. a specific threat

Your friend has an attorney to navigate his way through the chaos the ex is creating. He needs to work with the attorney, be open & honest, and get him to go to work on the problem. That's what he's paying them for. If not a criminal attorney, then his divorce attorney or whoever handles child custody.

If your family wants to feel safer, they're going to have to call the authorities if she threatens them.

Not much happy news, but hopefully news you can use.