As Doggy says, the LPR doesn't expire. However, it is a crime for an alien to be present in the US without a current card. It is an "issue" the day it expires if he's still present in the US. Of course, they're not likely to come looking for him to find out why his card is expired. He may find things get sticky if he gets involved with law enforcement for some other reason and this issue comes up. Obviously, trying to reenter the US without one is going to be sticky as well.

Why doesn't he want to renew? I know it's $450, but hell, it's only $45 a year essentially.