With all the money Microsoft makes, let them worry about catching software pirates, I'd refuse to do that for them. If your friend had doubts about ownership of the OS, the time to ask about that was before he paid for the computer. If he looks into it again, or contacts microsoft, they will have him install something like "Windows Genuine Advantage", which has been known to flag genuine licensed products as not being genuine, then even if you own the OS, good luck getting it fixed by MS.
I once had a dead computer and transferred some of the hardware and OS to a new computer I built. I then had difficulty activating the windows and Microsoft wanted to sell me a new license. I flipped out on them and demanded that the OS I owned and paid for be activated.
They eventually apologized and did so, but I bet they get a lot of people (like your friend) to pay them money, not because MS found it to be unlicensed, but because the consumer can't prove it is.