Married, Retired, and Living in Different States

GENERALLY, a joint return produces the lowest combined federal  income tax but the only way to be absolutely certain is to prepare separate and joint returns.

just because he is registered to vote in CA does not make him a resident

here are factors that Ca considers. 

The underlying theory of residency is that you are a
resident of the place where you have the closest
connections.
The following list shows some of the factors you can
use to help determine your residency status. Since your
residence is usually the place where you have the closest
ties, you should compare your ties to California with your
ties elsewhere. In using these factors, it is the strength
of your ties, not just the number of ties, that determines
your residency. This is only a partial list of the factors to
consider. No one factor is determinative. Consider all
the facts of your particular situation to determine your
residency status.

• Amount of time you spend in California versus amount
of time you spend outside California.
• Location of your spouse/RDP and children.
• Location of your principal residence.
• State that issued your driver’s license.
• State where your vehicles are registered.
• State where you maintain your professional licenses.
• State where you are registered to vote.
• Location of the banks where you maintain accounts.
• The origination point of your financial transactions.
• Location of your medical professionals and other
healthcare providers (doctors, dentists etc.),
accountants, and attorneys.

• Location of your social ties, such as your place of
worship, professional associations, or social and
country clubs of which you are a member.

• Location of your real property and investments.

• Permanence of your work assignments in California.

HI 

Residency Status
Resident
A resident is taxed on income from all sources.
A resident must file an Individual Income Tax Return—Resident (Form N-11), if required to do so.
A Hawaii resident is (1) Every individual domiciled in
Hawaii, and (2) Every other individual whether domiciled in Hawaii or not, who resides in Hawaii for other
than a temporary or transitory purpose.
An individual domiciled outside Hawaii is presumed
to be a resident if he or she spends more than 200 days in
Hawaii during the taxable year. This presumption may
be overcome by evidence satisfactory to the Department
that the individual maintained a permanent place of
abode outside the State and was in the State for a temporary or transitory purpose. No person shall be deemed
to have gained or lost a residence simply because of his
or her presence or absence in compliance with military
or naval orders of the United States, while engaged in
aviation or navigation, or while a student at any institution of learning. See Tax Information Release No. 97-1,
“Determination of Residence Status.”
Nonresident
A Hawaii nonresident is an individual who is in Hawaii for a temporary or transient purpose, and whose
permanent domicile is not Hawaii. 
Part-Year Resident
A part-year resident is an individual who was a Hawaii
resident for part of the year, and who was a nonresident
during the other part of the year. This includes those
who moved to Hawaii during the year and those who
moved away from Hawaii during the year.
A part-year resident must fi le an Individual Income
Tax Return—Nonresident and Part-Year Resident
(Form N-15), if required to do so. A part-year resident
will be taxed on all income from all sources during the
period of residency, and on income from Hawaii sources
only during the period of nonresidency.
Domicile Defi ned
The term “domicile” means the place where an individual has a true, fi xed, permanent home and principal
establishment, and to which place the individual has,
whenever absent, the intention of returning. It is the place
in which an individual has voluntarily fi xed the habitation of himself or herself and family, not for a mere
special or temporary purpose, but with the present
intention of making a permanent home. Three things
are necessary to create a new domicile: fi rst, abandonment of the old domicile; second, the intent to establish
a new domicile; and third, actual physical presence in
the new domicile. Once a domicile is established, the
intent to abandon it is not itself suffi cient to create a new
domicile; a new domicile must be shown.

my thinking is since he has established a permanent home in HI he's a resident

that means for CA purposes he could either be a part-year resident or non-resident.

this is probably a situation that should be discussed with a tax pro.