"It' broke" isn't enough information. You should be able to check the ranging, AC-DC converter and the buffer independent of the circuits using the LM311.
I would agree that your friend is right, there is a problem with the LM311 circuit. The LM311 has an open collector output, so it can only sink current. The LED has to go to +V through a resistor and the output will sink to ground or be an open circuit.
I also see at least 4 wires that don't connect to anything on the breadboard. Be sure you know how the breadboard works. In SOME breadboards, the long busses, such s the two on the left are 4 unconnected strips, not two.