It is possible that immigration will question him at the border and if they think he does not meet the residency requirements, they can start proceedings to revoke his PR. They will let him into Canada and he will have 30 days to appeal, either prove that he does meet the residency requirements or appeal on humane and compassionate grounds which could apply for example if he has been outside Canada because he was taking care of a sick parent or something similar.
If nobody says anything to him as he enters Canada, he only has to be careful that he does not apply to renew his PR card until he has 730 days. That means that he will have to let his PR card expire and wait a couple of months before he applies to renew. This should not be a problem for his daily life in Canada. If he applies to renew with less than 730 days, he is alerting immigration to the fact that he did not meet the residency requirements. If he waits until he has 730 days, he is safe.