How to reinstall from an USB stick on a MacBook or iMac?
If her MacBook was shipped with Lion, you couldn't be able to boot with a Snow Leopard stick. Apple Knowledge Base states that you shouldn't use earlier versions of OS X:


  Note: You should not use a version of Mac OS X that is earlier
  ("older") than the version your Mac shipped with.


If this is the case, you have to create a bootable Lion stick.