Is it possible to cache World of Warcraft updates, as my friend has a very slow internet connection and would like to copy updates from my PC

They used to have patches, but no longer function that way as it is handled by the Battle.net client now.

The easiest way to update for your friend would be to copy the entire World of Warcraft folder to a USB drive and copy to his computer, replacing the game files. Ensure that the Battle.net client is not running on his machine when copying the files. When the program starts, it will see the new data and verify the files on his machine before checking for updates.

Be sure to back up their WTF and Interface folders before copying to keep his addons and settings.