So let me get this straight ..??
1. Has some one created a fake XYZ profile and harassing your friend by posting offensive material on your friends wall etc ??
or
2. Has some one created a profile identical(imitating) to your friends and posting crap on it to harass your friend ??
..
Either way .. report profile to FB .. Option is there ..
get many people to do it .. 10-15+ .. So you might a quicker response.
As with rule # 1 of the Internet - dont try to block or hide it any content you dont like.. you will end up making it more popular.
The more attention you pay to it .. the more the culprit suceeds in his/her motives.
Ignore it + Report it ..
Although there are ways you can find out who is doing all this .. it would require that culprit to click on a link you send him .. that way you can find out a lot about his PC .. cookies .. IP ( city / ISP ) .. maybe even an email ID .. lots which can lead to discovering that person .. (if he/she has not be careful to cover his/her tracks) ..
Facebook wont give out a users info even in such cases unless instructed by a court of law .. Most what FB will do is delete the content(Profile) which is clearly in violation of FB Terms & Conditions.
EDIT: for more info