Sharp make a heap of "GPxxxxx" sensors that do the same, ie return distance to target.
The simplest way I can think of doing it though, is to have 1 IR transmitter on the person and 2 IR receivers on the robot, spaced apart with angular shielding. So the robot will turn and head towards the transmitter if it is to the side or ahead at a distance, BUT if the transmitter is too close the signal is shielded and the robot loses signal so it stops.
You could do the whole thing with a few junkbox parts.