There is no need to re-import. If the image files have been added to a permanent folder e.g. Pictures or My Pictures simply re-link to the existing location. Right-click on the desktop folder in the LR Library and choose “Update Folder Location” Then navigate to the folder containing the files. LR should automatically update the links.
Thank you for that, I will try and let you know if that has worked, thanks.
So let me get this straight.
The photos were copied from external to laptop, imported into Lightroom, deleted from desktop, copied from external and he wants to import again?
This is an very cumbersome and problem prone workflow. Start over. Make a new catalog, in Lightroom use File->New Catalog. Import the photos from the external HD using the ADD option in the Import dialog box. That's it, no moving, no copying, no deleting. It can't be any simpler.
With photo file sizes being what they are with the newer DSLRs, you can quickly use up your internal hard drive's space with images. Not sure how many images are being copied from the external to your friend's local drive but you may want to consider the implications that will have over the long run. Filling up your local drive can slow it down and if there is ever a hard drive crash, you might not be able to recover lost files.
You didn't mention why the images are being copied from the external to the internal drive. Is this a temporary fix to a failing external drive? These days with the cost of externals being reasonable I would recommend replacing the external with a new larger storage device that will last for some time. Keep the older external as an archive. Consider that new drive as a Main Photo Drive and go from there.