A "US National" is a legal term and is so defined in 8 U.S.C. §1408
8 U.S.C. §1408 Definition of U.S. National
...the following shall be nationals, but not citizens, of the United
States at birth:
(1) A person born in an outlying possession of the United States on or
after the date of formal acquisition of such possession;
(2) A person born outside the United States and its outlying
possessions of parents both of whom are nationals, but not citizens,
of the United States, and have had a residence in the United States,
or one of its outlying possessions prior to the birth of such person;
(3) A person of unknown parentage found in an outlying possession of
the United States while under the age of five years, until shown,
prior to his attaining the age of twenty-one years, not to have been
born in such outlying possession; and
(4) A person born outside the United States and its outlying
possessions of parents one of whom is an alien, and the other a
national, but not a citizen, of the United States who, prior to the
birth of such person, was physically present in the United States or
its outlying possessions for a period or periods totaling not less
than seven years in any continuous period of ten years—
(A) during which the national parent was not outside the United States
or its outlying possessions for a continuous period of more than one
(B) at least five years of which were after attaining the age of
IRS Definition of US National
An individual who owes his sole allegiance to the United States, including all U.S. citizens, and including some individuals who are not U.S. citizens. For tax purposes the term "U.S. national" refers to individuals who were born in American Samoa or were born in the Commonwealth of the Northern Mariana Islands who have chosen to be treated as U.S. nationals and not as U.S. citizens.
So US Nationals are people born in American Samoa or Commonwealth of the Northern Mariana Islands who decided that they don't want to be citizens of the united states. They refuse, and so based on that voluntary choice they become US Nationals. They can still get a US Passport, they can go to US and live there and get a job as a US Citizen. However they can not vote in National Elections. So US Nationals can't vote in National elections but besides that have the same abilities as US Citizens.
Giving up the citizenship of US means that the person is now considered a Chinese National, and would be treated by passport control the same as any other citizen of PRC coming for a visit, or on business and the will require a visa from a US Consulate in China to even board an airplane to fly to the US from China.
Since the definition of US National involves being born on 2 islands outside US mainland, while your friend was born in LA, they were considered a native born US citizen until the moment they gave up their citizenship and passport. From that moment on they became a citizen of China.