Mac air white screen won,t boot

Did you read the Apple support page on this problem? they are really good in helping: Troubleshooting My computer won turn on

won't get the check disc option until it boots up and from your description is is a hard drive problem
take it in for a proper diagnosis

if you can get hold of a windows disc try booting from that and using the repair option.

When you turn on the computer again, hold the Option (a.k.a. the Apple key) and a new screen should pop up asking with partition you want to boot into.

You may need to reset the PRAM.
Start your computer and hold down the option, command (apple), P and R keys simultaneously until you hear the second chime. Then release.

This will reset the perimeter RAM (PRAM) and may allow your system to start, which it will do immediately if it capable.
If this is not the case and you have already tried removing the RAM and the HD, you may need to have the motherboard tested.

Leave the Macbook air turned on until the battery goes dead. Then Connect to power and start it up. That should solve the problem. You problem was caused by not letting an update install in full.

The flashing folder means that the computer can't find the boot system on the hard drive. If your girlfriend's step-dad has removed some parts of the OS, then the system will not start giving the 'flashing folder' result. You will need to re-install the OS from the system disk.
Insert the disk into the computer and it will boot from that and take you through the install procedure.

Try to boot your computer with the Recovery disc, by turning on your computer and slot in the disc, and pressing the C key. If it succefully boot in, try to restore the Mac software.

you have a mac with a program called boot camp. What you need to do is to hold the option key down once you restart it. Keep holding it down until you get to a screen that shows two hard drives on it, one says mac and the other says windows. Use the mouse or arrow keys to choose the mac icon and then hit enter. and yes, you will have to do this everytime you reboot.

Could be hard drive crash.. Try loading MAC OSX from CD to see if it will fire up.