Mate have a read of this site:
http://www.fuelsaving.info/debunk.htm
He's a engineer who speaks the truth about all these clip on mods.
He even details the magnets they put around your air intake:
http://www.fuelsaving.info/magnets.htm
Your mate has been ripped off and now has a rather expensive fridge magnet.
From the site:
"Most "fuel saving devices" fit this pattern:
About a 10 - 15% claimed fuel saving (gas saving / gas mileage improvement)
Claimed reduced emissions
Claimed improved performance
Cost about £50 (80 US$) (so costs about £5 to make!) "
This isnt saying that I have tried it and it doesnt work. Just fits the typical clip on 'mod' that does nothing except lighten the load on your wallet.