"...He is insured on the car with his (now expired) international license. I'm not sure what the rammifications of this are..."
The ramifications would be that if he got into an accident, when a claim was filed, your insurance company would see (from the accident report) that his license was expired, and they would say, "Sorry, we don't pay on this. He would only be covered if his license was valid/in force."
Bucyn made some very valid legal points; you should look into what trouble you could get into.
NoelleR