Had your friend moved some Time Machine backups to the trash? If so, that's a problem (and has likely corrupted the backups, too).
If so, try emptying the trash while holding down the
Option key.
If that doesn't help, Open the
Terminal app (in your Applications/Utilities folder). Be *very careful* with this app. It's a direct link into UNIX, the underpinnings of OSX, but without the protections of OSX.
In Terminal, the prompt looks like this:
user-xxxxxx:~ <your name>$
(where <your name> is your short user name). It's followed by a non-blinking block cursor.
Type the following
exactly as shown, placing the name of your Time Machine partition
between the quotes, and be sure to place spaces where shown:
cd /Volumes/"Time Machine Backups"/
and press
Return. The prompt should then change to include the name of your Time Machine drive. Then copy or type:
sudo rm -rf .Trashes
and press
Return. It should then ask for your Admin password (which won't be displayed). Press
Return.
If there were many items involved, this may take quite a long time. Unless there are problems, once the deletion is complete, you'll see another prompt (there's no completion message).