Ok here we go:
1- You have to restart your MBA holding down the Command+S keys, and this will take you into Single User Mode and it’s Terminal interface.
2- Then you have to check the filesystem. to do this you have to type the following command in the terminal interface:
3- Then you have to mount the root drive with write option enabled so you can apply and save any changes. Type this in the temrinal interface:
mount -uw /
4- And then type this command exactly as you see it here:
5- Reboot your MBA by typing this in the terminal interface:
6- After you reboot, you will be see the “Welcome Wizard” startup screen. Follow the wizard and create a new user account. This new account name must be different from the onewhose password you want to recover.
7- Continue and boot into your Mac OS X with the new account you have just created, this new user account is an Administrator and has administrative access
8- Now that you're logged in, go to System Preferences
9- Click on Users & Groups
10- Click on the Lock icon and use your newly created user name and password if asked. This will allow you to make changes to other user accounts
11- On the user panel select the user account whose password you have forgotten and then click on the Change Password... button.
12- Enter your new password (you try not to forget it again )
13- Reboot/restart your MBA and now you can log back in with your old account. If you want you can delete the user account you created following this steps.
Hope this helps.
The problem is that if you force a password reset in terminal with the command+r option then your Keychain access and your account login password won't be synchronized anymore. That's why my steps are a better way to recover the password even if it is a bit more complicated. If he resets his password in terminal with the resetpassword option then he will have to go to delete his old keychain , create a new one and believe me, it will take longer.