Of course you can. I have seen many doing it without a problem occured. However, since it was placed on your friend site then you have no control over them anyway. If those ads got a very low earning per click then you cannot do anything with it unless you have your friends site FTP and a persmission to change his webpage. It's up to you.
yes, you can if your friend allow you to put the ads on his/her site. However, you need to bare the risk, mean if your friend website against any adsense TOS, your account will get banned
can you? yes, you can should you? i would not suggest. i have seen friendship turning sour and what people used to call friends once turned into more than foes. so called friends click bombed and got lot of accounts banned. will not happen always but better safe than sorry when it comes to financial transactions. just my 2 cents
I would not suggest to do this since adsense is very clever to find out such stuffs. Recently my friend's account got banned just because he placed his ads on different sites.
oh.. yes, we have no control over other site content or invalid clicks and google may banned account...
As long as your friend's site is in compliance with the Adsense Program Policies, it's fine. But you shouldn't just trust your friend to be following all the rules. You need to review the site and the site's sources of traffic to be sure your account won't get into trouble.