Warranty must be honored by UK law, regardless of what's printed on the warranty card.

Yes, but the Karma gives you 1 year's manfacturers warranty. Under UK law you statutary rights are less than the 1 year's warranty. There is nothing to say that Rio UK have to honour the extra warranty on goods imported from somewhere else without the additional warranty included.

I'm not saying they won't honour it, but if it is a US sourced Karma they are within their rights to refer the customer to Rio US and its 90 day warranty.

It wouldn't be worth the risk unless it was very cheap.

If he had wanted to take that risk he could have bought it from the US and probably got it for £200 rather than the £270+ he probably paid for it. They should give him his money back, legally and morally.