Welcome to WebmasterWorld Guest from 188.8.131.52
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.