Even in CA, the “three pillars” still apply: ability, opportunity, & jeopardy. No matter how much people want to point to “stand your ground” or “castle doctrine”, it still comes back to these three, including with your own conscience (if it’s well formed). Could you live with killing Robert Downey Jr. because he broke into your house while high simply because, hey, “castle doctrine”?
Ability: Can you convince a jury that your “assailant” was capable of causing your death or great bodily injury? Were you able to give LE a description of the type of gun/knife/weapon he had. There’s a big difference between “He had a Glock 17 pointed at me.” and, “It looked like he had a gun.” Or was he 6" taller, 50lbs heavier, and wearing a UFC shirt? You have to be certain about convincing a jury that the person had the capability of causing you death or great bodily harm if you didn’t use deadly force.
Opportunity: Is the “assailant” capable of immediately using that ability? This includes your ability to evade/escape. This is where the “21 foot rule” often comes up if the person had a knife or bat, etc. The threat needs to be “immediate”, so you didn’t have any other alternative. Can you convince a jury that the threat was already upon you and you couldn’t just drive away or lock a door between you and the other person, etc. If you can draw and fire two shots within 2 seconds, were you two seconds away from being harmed? Could you have done something else to avoid it or was shooting your only option within that amount of time?
Jeopardy: This can be a tough one because you have to convince a jury that he had the intent to kill you or inflict great bodily harm. Someone running towards you proves nothing. But someone running towards you with a weapon, yelling their intent to harm you is a different situation. Do you know the person? Have they made threats before? Or were they about to run past you to get to their car because they just got a call that a loved one is in the hospital?
Also in CA, there’s a big difference if this happens in San Francisco vs if it happens in Lone Pine, for example. If you use a handgun to defend yourself in SF, you’re immediately the “bad guy”, you’re going to jail, all your firearms will be confiscated and vanish into thin air, and your jury will be anti-2A activists who are upset that they couldn’t get out of jury duty. Good luck convincing someone who thinks eating meat is animal cruelty, that you had no option but to use deadly force to defend yourself against a fellow human.
I’d suggest getting your family member a book by Massad Ayoob called, “Deadly Force, Understanding Your Right to Self Defense”. Mas has a pretty lengthy background in what takes place in “use of force” cases in courtrooms throughout the US.