The New York State DMV says this:
Out-of-state and foreign driver licenses\
You can drive in New York State with a valid driver license from another state or country. You don’t need to apply for a New York State driver license unless you become a New York State resident.
If you have a driver license from another country you do not need to have an International Driving Permit, but it is helpful. The permit verifies in several languages that you have a valid driver license. Police officers who can’t read the language on your foreign driver license will be able to read the permit. Contact the authorities in your home country to get an International Driving Permit.
If your friend doesn't live in New York State, then the police officer was mistaken and issued a wrongful fine - your friend can have the fine waived due to the guidance issued by the state DMV, as cited above.
For information on how to get fines waived or tickets rescinded, you could ask a question on the Law Stack Exchange.
If your friend does live in NY State, and the state defins that as intending to live at any address or addresses in NY State longer than 90 days then you need to seek legal advice - NY State exclude foreign students from the residency definition, but whether that includes J1 visa holders is a question I cannot answer with certainty.
I would suggest it does, as a J1 visa is dealt with in the same areas on US gov websites as the F1 visa, and links to the F1 and M1 visas under "other student visas"...