As Al said, it is my understanding that Surefire did not originally have a rail adapter for the USP, but one was in the pipeline. Best bet would be to talk to Surefire.
I've seen a side by side comparison of the HK light and x200, and personally I prefer the x200. If you do a search on cpf for the x200 you will find that the only major drawback is that there is less peripheral / ambient light than a corresponding incandescent light. I don't actually remember finding anything on cpf about the HK light...
Depending on your purpose in my opinion the x200's lack of ambient light is outweighted by the other positive features such as that the LED that is essentially impervious to recoil and is not (to my knowledge) subject to failure like an incandescent bulb, its small and compact size etc. But of course this comes down to your friend's intended use for the light.
Just my .02