You need to post to your friends feed, Easy way would be to adapt your code to use the appropriate graph method with the persons you wish to post to facebook ID (i've assumed here you have stored this from an earlier call the graph api in person.facebookID)
[facebook requestWithGraphPath:[NSString stringWithFormat:@"%@/feed/",person.facebookID] andParams:params andHttpMethod:@"POST" andDelegate:self
http://developers.facebook.com/docs/reference/api/post
To get a list of the users friends use the graph path me/friends
[facebook requestWithGraphPath:@"me/friends" andDelegate:self];
Which will return a JSON list of the users friends names and their Facebook ids, to the appropriate delegate method from FBRequest, its probably worth wrapping this set of results into a set of person objects or storing the returned NSDictionary so that you can retrieve individual friends, its upto you how you process the list of returned friends (you probably want to use a UITableView to display the user friends or filter based on some other input from the user)
Using this method will mean you do not have to use the Facebook dialogs in the iOS sdk, which does mean there is an upper limit to how many messages the user can post in a day using your app
If you wish to use a dialog you will need to include the "target_id" in your params dictionary and set this to the persons Facebook ID you wish to post to
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