Transfering merits

This is a tradition I have heard in Thailand: Many men become monk for a few months in order to transfer merits to his parents and so they can go to heaven. If I'm not mistaken the Buddha said you could only transfer merits for ghosts (according to teacher Dhammavvuddho Thero), it is not possible to transfer merits from man to man. Is it? Any text to help? I may be wrong so I appreciate the explanations.