A friend of mine is travelling to the states and wants to tour around SoCal on a motorbike. He has his motorcycle license in New Zealand. Is it legal for him to ride in California? Are there any states in which it's not legal for him to ride? Will he have to buy special insurance?
The relevant law is California Vehicle Code §12502 and §12505.
According to §12502,
(a) The following persons may operate a motor vehicle in this state without obtaining a driver’s license under this code:
1. A nonresident over the age of 18 years having in his or her immediate possession a valid driver’s license issued by a foreign jurisdiction of which he or she is a resident, except as provided in Section 12505.
According to §12505,
[...] residency shall be determined as a person’s state of domicile. "State of domicile" means the state where a person has his or her true, fixed, and permanent home and principal residence and to which he or she has manifested the intention of returning whenever he or she is absent. Prima facie evidence of residency for driver’s licensing purposes includes, but is not limited to, the following:
(A) Address where registered to vote.
(B) Payment of resident tuition at a public institution of higher education.
(C) Filing a homeowner’s property tax exemption.
(D) Other acts, occurrences, or events that indicate presence in the state is more than temporary or transient.
[...] (c) Any person entitled to an exemption under Section 12502, 12503, or 12504 may operate a motor vehicle in this state for not to exceed 10 days from the date he or she establishes residence in this state
What that all means is that, yes, if you have in your possession a driver's license issued by your country of residence, and it legally allows you to operate a motorcycle, you can use that foreign driver's license in California to operate a motorcycle.
There is also a decent writeup by not a lawyer at California Driving: A Survival Guide