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.

Yes, and I've done it.

The game files, which are the huge gigabyte patches they regularly do, are stored in a single folder in the game's directory. Called /Data/ if I recall.

Here's the thing. The game doesn't mind if you patch that directory behind its back.

I know this because I used to run three WoW installations (for different add-on configurations), but I didn't have 120GB to spare for three full installations. So I used sym-links (not sure what they're called in Windows) to point all three /Data/ directories to the same place (an external drive, actually).

What happened is, when an update came out, the first installation I chose to run would sit there and fully patch. When I ran the second installation, it paused for a few seconds and went "Oh ok." and proceeded. All it needed to do was confirm the patch was already done. It worked great.

So you only need to copy the /Data/ folder. Don't mess with the other folders because that will affect your game settings, add-ons and their configuration. (and anyone who runs TradeSkillMaster or other complex add-ons does not want to do that!)