In this part of the world,most of the international transactions were perfected with debit cards because the credit system have not been fully developed. This is no exceptional case! By the way,have you conducted sample poll to be sure and to have come to the conclusion that debit cards transactions are declined.
Did your friend find out if his card was enabled for international transactions? How much do you know about debit cards/credit card working mechanism? What is the difference between debit cards and credit cards? Please this is a public forum and information is key,when you are not sure of something don't come on here and spew it all over the place!
Your friend's case is an isolated case and by what you wrote,you couldn't have concluded that his application was rejected as a result of of his using a debit card.