While you can put your friends AdSense code on your site you should not as Google will most certainly close your friends account when they detect this and detect this they will. They may give him a chance to take down the adsense code before suspending him but that is not very likely. When Google catches this they will also likely deem all of the recent AdSense activity under your friend's account as fraudulent and refuse to pay out any outstanding funds!
You (and your friend) should read the terms and conditions your friend agreed to when he was accepted to put AdSense ads on his site. I would suggest that instead of trying to go around their approval process you should try to make your site acceptable to Google so they will approve you for an AdSense account. I know they can be very frustrating to work with at times but assuming you have a legitimate, beneficial site that isn't duplicating a Google provided service, usually issues like yours can be worked through.
The exact terms and conditions depend on which Country your friend is located in but here are a couple of the sections of the US agreement which addresses this:
AdSense for Content. All content and Property-based Ads (and Ads served in response to end user clicks on and queries entered into Links, if any) shall be grouped by Google and displayed with Links (where applicable) to end users of the Property(ies) as ad units (such groups of Ads and/or Links collectively referred to as "Ad Units") in standard formats as offered generally by Google from time to time, as may be described in the FAQ. You may select a format approved by Google for the display of Ad Units in connection with the Property(ies), but You acknowledge and agree that Ads and/or Links: (i) shall only be displayed in connection with the Property(ies), each of which is subject to review and approval by Google in its discretion at any time; and (ii) shall be subject to the placement guidelines set forth herein.
Google may investigate any activity that may violate this Agreement. Google may at any time, in its sole discretion, terminate all or part of the Program, terminate this Agreement, or suspend or terminate the participation of any Property in all or part of the Program for any reason.