I can't find any Hawaii State law reference prohibiting me from sending via USPS a rifle to my friend on a different island.
I would be loaning him the rifle for no more than 14 days.
From ATF site:
May a nonlicensee ship a firearm through the U.S. Postal Service?
A nonlicensee may not transfer a firearm to a non-licensed resident of another State. A nonlicensee may mail a shotgun or rifle to a resident of his or her own State or to a licensee in any State. The Postal Service recommends that long guns be sent by registered mail and that no marking of any kind which would indicate the nature of the contents be placed on the outside of any parcel containing firearms. Handguns are not mailable. A common or contract carrier must be used to ship a handgun. [18 U.S.C. 1715, 922(a)(3), 922(a)(5) and 922 (a)(2)(A)]

I have asked this question to various FFLs and received various answers. As a former FFL (gave it up in 2000) I have not kept up with reg. changes; however, It is my understanding that this action is permissible

Can anyone give me a definitive answer citing printed law and not just opinion?

US Mail: An unlicensed person can ship a rifle or shotgun by US Mail. Unlicensed persons cannot ship a handgun by US Mail. Pistols, revolvers, and other firearms capable of being concealed on the person (for example, short-barreled shotguns and short-barreled rifles) are defined as handguns Postal regulations allow the Post Office to open your package for inspection. Ammunition cannot be shipped by US Mail.

Thank you for responding. I am aware of the postal regulations concerning the mailing of firearms.

Specifically I am wanting to send a Rifle from 1 non licensee to another non licensee who live on 2 different islands.
If we both lived on O`ahu I could drive to his home and hand him the rifle.
He must return the loaned rifle within 15  days according to Hawaii Law.

Federal law (see OP) requires non licensee to ffl ,when sending out of state, but permits non licensee to non licensee within a state.

My question is: does Hawaii State law prohibit shipping/mailing non licensee to non licensee within the state?

NO
HRS does not restrict private transfer or (loan). All you need to be sure is he has a permit to aquire, proof is NOT required. and
both parties must not be a prohibited person.

Thanks for the reply.
That was my understanding; however, I have gotten different answers from current FFLs.
I was told I needed to send it to an FFL and a FFL needed to send it back to me.

I see no mention in any of the Hawaii statutes other than:

 Shotguns, rifles, antiques, and muzzle-loading firearms may be shipped by U.S. Mail.
Rifles and shotguns may be loaned for a maximum of 15 days to persons lawfully able to possess. 
Private persons may not send firearms across state lines to one another (unless they are federally licensed firearms dealers) except to and from a factory or authorized gunsmith for repair or to ship inherited firearms.

Does not mention restrictions shipping within the state.

basiclly u can loan him the gun legally. he can apply for a permit to aquire while its on loan to him.