If the car is going to be left for that long, I would recommend disconnecting the battery, and if the garage is level to park with the handbrake released. Just chock the wheels with something.
It also good to wash the car and pump the tyres with air before parking and check all the fluids. Its also best to just have a 1/4 tank of petrol in the car as petrol goes stale after a while, then fill it up once he is back.
Once he comes back after 6 months, before starting he should do some checks, see if there are any leaks under the car, check the oil and all the belts and most importantly check the coolant all over again.