I told my fellow worker about how VGC updates old sale charts and how NPD chooses not to release those updates to the public.

 And He retilated by saying that VGC completely guesses the European sales numbers. So how accurate can there worldwide sale charts be if they completely guess these sales numbers?"

What's your insight and input on what he had to say? 

Right here is where your friend epically phails.  VGChartz gives data for WORLDWIDE sales, whereas NPD is USA ONLY.  OF COURSE VGC is going to give higher numbers! 

Your friend is wrong, we do have actual data as the basis for our European numbers.  But he's a total fanboy anyway so rational arguments and logical reasoning aren't likely to work too well on him.

I've never heard of NPD updating old charts before.

 Joke? The numbers for both are US only, but the guy's friend added in an extra week for the VG Chartz data, something no one seems to have noticed yet. VG Chartz was way off for DS for January, either way.

You could tell him that he is completely wrong. For one the site is based in the UK (i.e. Europe).

Better yet ask him to read this thread. If he still says VGC is BS after that, then forget about it. Just let him live in his world of green skies and yellow snow. 

 Or having any European data whatsoever, for that matter.

Is it just me or are those percentages way off?

If Im not going crazy it should be:

Wii (-6.76%)

PS3 (+3.21%)

360 (+14.02%) 

Now that you mention it yes DevilStation's %'s are off. Perhaps I should have double checked his numbers before being lazy and stealing them =P

Either way with the possible exception of the 360 the %s are still close enough that you can legitimately say they agree. The 360 being off by 14% only means VGCs data is sufficiently different for us to say that the two data sets disagree. Without a third set to calibrate against its pretty hard to say who is actually closer, again its all about margin of error.

Tell him that for worldwide sales figures the only sources with any degree of accuracy are VGChartz, which uses the same methodology as trackers such as NPD, Chart Track and Enterbrain but on a smaller scale or in shipment figures, which only come every 3 months.

Because of the way European sales numbers are released to the public by the major regional tracker (Chart Track) they are impossible to use. This means the only actual tracker that releases usable European sales data to the public is VGC.
Combine this with the fact that compiling NPD, Media Create or Enterbrain data is useless as they never release any of the modifications they make to their data to the public, the data they release can only be used for that one month and compiling multiple months together to give an indication of sales is highly misleading.

Essentially for the individual periods of release the data for NPD, Enterbrain and Media Create is more accurate than VGC data but any compilation of the data is less accurate.