I have a good friend in SoCal who contacted me about what parts he needs to get to put one together. Now, its been nearly 25 years since i lived there so not quite sure what he actually needs today. Sure i know how to build one, and have built myself many, so i have a good idea parts wise. But compliant parts im no expert. If he has a bullet button can he retain the collapsing stock? Or an a2 style stock will he need a bullet button at all? Pin and weld compensator?

In a nutshell.  Best bet is to look them up on an authoritative sight, look for a California Rifle Flowchart, Calguns, etc.  (I think they have a typo on their chart, btw, not sure it's a problem but would need to scan the Penal Code to be sure.  Which you may want to do anyways.) These are the things you can't have for featureless:
   Pistol Grip
   Thumbhole Stock
   Folding/Telescoping Stock
   Grenade Launcher or Flare Launcher
   Flash Suppressor
   Forward Pistol Grip

Barrel length must be at least 16" and overall length must be over 30"

With these features (or lack of these features) you can have a traditional magazine release.  No magazines over 10 round capacity.  There is ongoing discussion about stock features or if a stock is a pistol grip.  the law/legal definition is unproven/tested in court so far.  Also, while it's pretty straight forward to not include a flash suppressor, whther other types of muzzle devices are legal is also somewhat unclear as the definition there is getting discussion as well.

I haven't looked into the break action options/opportunities.

Thanks. I'm trying to get a list together for him so he purchase what he needs. Ive seen the flow chart and its wholly confusing, I'm sure by design. As far as configuration, probably better he went with the idea of a precision AR or Grendel. the stock will play into that not sure how you get a good cheek weld or correct LOP with some of these contraptions out there.

Reading this just makes me sad.  I know it has come about piece by piece over a number of years but if you take a step back and read the reply from the point of view of someone outside of California who is not used to all the crap the state is putting it's citizens through it is depressing.  I live up in Washington now and haven't lived in California since before I joined the military in the 90's but I know that unless the federal government steps in the California rules will be coming to Washington soon.  they are currently working on a new law up here that will require us to keep all magazines unloaded and stored in a safe.  Just know that many outside California do what we can to support you. Keep fighting the good fight.

I know. I grew up in California and remember going to the many very cool gunshops around. Kings anyone? I recall seeing full auto pre 1986, and I remember shooting in an area around wrightwood where folks would gather and shoot their full auto goodies. Now? OMG, its saddening to me. But I'm going to do my pal right. Its either he's going to build an AR or just get a mini-14...or both? lol

Thordsen has a couple of approaches to the cheek weld issue as they have both the rifle and pistol style buffer covers.  The Command Arms cheek rests for the M4 style stocks fit the buffer covers kitted with some of the rifle stocks and the Mission First Tactical EVOLV Battle Stock Attachment is kitted as a part of their approach to a buffer cover for an AR pistol.  I suppose one could "layer" the cheek rests if needed or desired.  I believe the Thordsen has been used in New York as SAFE compliant so there may be some users in the NY forum that have some thoughts as well.  They also have a thinner buttplate available and I believe I read that it's the same size as one made by Tapco? so that may offer some options for depth or to use as spacers?

I'm not sure the flowchart is intentionally obtuse but maybe it just got worse over time while they tried to keep it to one page?