You need to hook into the register, menu:topbar hook and remove the item with the name "friends."

Your callback would like something like this: https://gist.github.com/1888009

FYI: I found this information by checking the HTML. All menus in Elgg have classes that let you guess the menu names. If you look at the menu item's <li> tag you'll see the class "elgg-menu-item-messages". Look up the DOM to the <ul> tag and you'll see the class "elgg-menu elgg-menu-topbar elgg-menu-topbar-default".

These classes mean you're looking at the menu item named "friends" in the "default" section of the menu named "topbar".

Brett, wouldn't it be wonderful to have another hook before 'register' in the factory, which would allow to use register/unregister api, without having to iterate through the return array on 'register' to find the menu in question?

@Ismayil - I actually forgot about the unregister function. In this case you CAN use it because the friends menu item is registered by core. So you can replace the hook with elgg_unregister_menu_item('topbar', 'friends'). That's much simpler :)