v2.0でこれじゃだめかね？v1.0版の/user/mutualfriends/のupdate版みたいだけど
@current_user.api.get_object("/#{params[:user]}?fields=context.fields%28mutual_friends%29")
<https://developers.facebook.com/docs/graph-api/reference/v2.0/user.context/mutual_friends>