"Every air traffic control frequency is under the control of the ATCO who is providing the service on that frequency. A pilot would virtually never speak directly to another pilot, although that is simply a matter of pressing the transmit button. Usually what happens is to speak through the ATCO i.e. "Tower, doesn't the 777 on finals look a little low?" This serves to bring everyone's attention to the situation and the Controller might well respond with a call to the 777 such as "Asiana 214, you look a little low, are you OK?"
I was wondering the same thing. Isn't ATC required whilst on an IFR flight plan to give you low altitude alerts (7110.65U 2-1-6)? "a. Terrain/Obstruction Alert. Immediately issue/initiate an alert to an aircraft if you are aware the aircraft is at an altitude that, in your judgment, places it in unsafe proximity to terrain and/or obstructions."
Or does their obligation end just because they cleared you for a visual approach. When I was working on my rating I'd gotten nudged on practice approaches in VMC/VFR under the hood.