Why not plug in the heater 24/7. If your lights and the ambient temp keep the tank at or above the set temp then your heater shouldn't be using any power anyways. Set it on the low end so it only has to kick in when temps drop to uncomfortable levels for the fish.
Exactly right...
Sorry though, about your fish. Unrealistic to think he's going to remember to plug in the heaters when he gets hot if he doesn't care about the fish himself, though - so best to just leave them on, dialed down to a low setting.
If he is in charge of the fish while you are gone, I can see giving him such instructions, but if he's just your roommate and doesn't give a hoot about your fish tanks...well. That's different!