In the thermostat I use for my spa, using an LM324 op amp for the trigger, my feed back resistor is pretty big (1m pot) so the on and off points are pretty close together. +/- < 1 degree f on a thermistor sensor (automotive water temp sensor)in a Whetstone bridge with two 10ks on the far side.
We really need to know more about what kind of photo sensor he is using and what the outputs are. I agree a latching relay is probably the lowest part count solution if he can drive it from the sensors and he wants one to always be "turn on" and the other one always "turn off". You can get latching relays that operate around 1 ms so if this is not really something where things are screaming by, it should work.
