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If your friend has got a Late 2010 MacBook Air, the Mac came with a USB drive that contains Mac OS X Snow Leopard, to reset passwords, reinstall Mac OS X...
If he/she has got an older MacBook Air, he/she has got DVDs instead, and you need an external optical drive or a PC to be able to reset the password. See > http://support.apple.com/kb/HT2129?viewlocale=en_US&locale=en_US After starting with the DVD, open Utilities menu > Password Reset, and follow the steps
If he/she lost the USB drive or DVDs, call Apple to get replacement DVDs or another USB drive > http://support.apple.com/kb/HE57