Hi Chris66:
Welcome!I'm so sorry about the situation with your son's partner. It sounds both dangerous & serious. Do you live in the U.S.?
Anyone pulling a gun on you is unacceptable & dangerous. The gun and any other weapons she is apt to use have to be taken away/removed from the home! Weapons can't be available to anyone who exhibits behaviors like your son's partner.
The last few months she has gotten worse, pulling a gun on him, calling police claiming he is abusing her only to retract what she claimed, threatening to take his baby from him and leave the state, screaming and falling to the ground in public, threatening to ruin his life by claiming he is abusive so he can't see his daughter, and so much more. . .
He has been to lawyers, police, and counselors for help and all they say is he has to have proof she is unstable in order to get custody of at least his baby and to help his step kids go to their dads
You son needs to quit letting his partner intimidate him & get serious about the situation and document all bad behaviors & events. Fear of his partner shooting him or harming the children in some way, should far out weigh his fear that she will get full custody.
In the long run, presenting sufficient documentation can end up forcing his partner to get mental health care. (since she refuses to seek care) I don't know if it is legal where your son lives to record someone, but your son should consider recording his partner in an effort to document some things like death threats. Even if a recording can't be officially used in court, I've heard where recordings have been used to prove to police responding to an emergency call to your house, that you are the victim. Also, it would be helpful to record her threats of making false claims of abuse.
You might want to post on the "Family Law" board. You will likely find some support there about documenting for legal action.
You can help, by making copies of text messages and emails. Documenting in writing about events you are aware of & what was said. Basically, a journal - documenting date, time & what happened. It's best, if you witness certain things first hand, but it still could help to document the occasions you get calls from your son for your assistance and the reasons why.
If you son join's in with documenting (he really needs to), he will need to be very careful to secure his documentation so his partner can't gain access in any way. Perhaps you can be the depository for documentation (i.e. you keep the thumb drive & make extra copies).
Your son needs to realize that he has to get busy with documenting. Things can't get better until he does. A good place to start, is to make a plan for documenting. As best possible, go back in time and document things like the day the gun was pulled. Phone call records & texts could help.
The guns need to disappear.