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Do you realize that if your friend gets illegal clicks on his site that it can disable your entire account?
Why does your friend just not add AdSense himself/herself?
The questions are rhetorical. My best advice would be for you to approach him with a business proposition. You would consult with him on monetizing his site more effectively for a certain percentage of the ensuing profits. This way he would be the one to open the account and it would be 100% kosher by Google standards.
You may also want to consider buying the site outright if you feel that there is untapped marketing potential there.