Having checked the code in the mentioned file I assume that this error might occur if you try to delete a review of a customer whose customer account has been removed. I have not verified this - I am to lazy to create some customers and reviews or install the sample data…
Be that as it may - if you know a tiny bit of PHP simply go to the file /app/code/core/Mage/Adminhtml/Block/Review/Edit/Form.php. There add the line
$customerText = ‘unknown’;
directly at the beginning of the function _prepareForm.
That should be it.
Please note that I have not tried this, so no guarantees. And as always - make a backup of the file before you change it.
PS: If this should fix the issue, could you please post this fact here? This way other people might find the solution