Normally there is phone number in the letter. You friend can call Revenue Canada to ask about what is going on. He should never ignore the letters from Revenue Canada. If he owes money, he needs to pay it back(plus interest)
nothing is wrong.
the Revenue Canada is just working on their routine. Your friend's file might not be updated, they assume that he is in Canada. So, your friend need to call and inform them that he left in 2005.