I hate to be the possible bearer of bad news but your friend may have even wiped (deleted everything) the drive prior to re-installing. If there's nothing of value on your drive then you've nothing to worry about other than trying to get an OS the hardware supports back on the hard drive.
Whenever I've seen this happen it's usually because someone has tried to 'upgrade' the existing OS first. This approach fails because the Lion Installer will say something like "the hardware does not meet the installer's minimum requirements". At that point your friend should have handed it back to you and it would effectively have remained untouched and hopefully still working as it was before you handed it over. However your friend may have got confused by the failed installation and attempted a reformat and install. He/she may have even attempted this next step without backing up the data that's already on the hard drive first. By the time he/she have formatted the drive and gone back to the installer it will be too late. I hope for your sake you friend has (a) backed it up and (b) not formatted the drive. There is also a possibility the installation failed because of long-standing drive/directory issues which then disabled the existing OS. Hopefully this could mean your data should still be intact and safe.