tell them to read their policy. it will spell it out.
(i'm in CA, my insurance is AAA)
for example: my car if another driver drives it, my car will be covered in case of accident if the driver of my car is at fault. this covers insured and uninsured drivers. when you're pulled over i believe they only ask for proof of insurance and not your own personal insurance anyways. there are two people excluded from my policy - my dad and my bf since he's considered a "regular" risk. my dad, it doesn't really matter, because he also has AAA insurance and is also insured for any car he drives. my bf's insurance doesn't have that coverage, so if he drives my car and crashes it AAA will not cover it and neither will his insurance.
the guy should still have car insurance since he's "leasing" a car. he's freeriding right now...