Yeah if the ECU was shutting the tractor down when the sensor when out it likely was important for proper operation or safety. For the most part if it is an unimportant sensor the new machines will complain about it but let you continue operating it, perhaps with the system controlled by the sensor disabled or limited in some manner. Similarly to how a new car will show a warning light or enter limp mode for minor sensors being out but refuse to crank for more important ones.
Everyone thinks "oh its a tractor, it is simple" but these things are very, very far removed from the things you would see in quaint rural settings shown in movies. It is an extremely complex, powerful, and dangerous machine and they do kill people with depressing regularity. The controls for the hydraulic system are something you really don't want to have people monkeying around with as overpressure/overtemp could cause damage to the implement and rams or even (and I've seen this happen) a leak the engine compartment which sprays onto the exhaust manifold totals the machine at best or kills the operator at worst. Underpressure could cause the implement to drop unexpectedly and dump a few tons of steel and blades on an unfortunate worker or cause overheating (as some systems use the hydraulic system to run the engine fan).
The real problem in this is that the sensor keeps going out for whatever reason. Deere parts aren't cheap but he should talk with his equipment dealer about having a spare on hand at the farm that he can swap out himself if it fails. Takes two days waiting for the part down to an hour or so to swap it.