http://www.dmv.ca.gov/pubs/brochures/howto/htvr9.htm
You only pay Sales ("use" actually) tax if you purchase the vehicle less than 365 days before you relocate to CA - in your friend's case it will apply since regardless of whether he keeps it registered in OR for a year, if he is already a CA resident, there is no way around it.
i.e. if you were resident in another state & have owned the car for more than one year prior to relocating, there is no tax to pay.
(This was circumstance when I moved to CA from TX - paid no use tax on 2 vehicles I transferred when I relocated)
But in this case the use tax would apply, no different that buying a car in CA for that particular component.
Car needs to be CA smogged as well so one more additional fee Todd did not mention - there are also some misc fees if you check the fine details
It's all laid out in the above link, as is everything else you needed to know about auto registration & drivers' licences etc - most informative site!
Bottom line is the only additonal costs vs buying a car in CA are an additional $15 service for an out-of-state vehicle and the smog fee - if purchased in CA the seller is responsible for the car to be smog compliant, not the buyer.
Oh ..... & since it's an NSX he'll need a personal plate fee!
Have your friend follow the rules - it just costs the rest of us more when the rules are bent.