Probably moisture getting in under the clear coat finish. If he rubs them back he will need to respray them with clear coat otherwise the aluminium will oxidise quite quickly and go dull.
My nephew had the same problem and they where replaced. He was told that there was a problem in manufacturing and the wheel was flexing which caused the problem. It may or may not be the same thing.
I've noticed the wheels on our 2012 van doing the same, including the spare which has never been on the road, so I doubt flexing is the cause. When it bothers me I'll rub the wheels back and re-coat them.