Is the disk that you are writing to also a bootable disk? In other words, is this the disk she uses to startup the mac from?
If so, you can't write to that disk without messing up the permissions. The general rule of thumb with any bootable disk is only ever copy from it when you're not booted into it. Changing the permissions from your end does not get around this. If you write into another bootable disk from 'outside' of it, you'll just corrupt the permissions for that user when they start up from it.
If you want to make things available to her from your disk, put them in your 'shared' or public folder, or even easier is use Dropbox or Skydrive.