How long can oil sit for?
If oil is even in contact with the wires the unit is defective or has been modified, making it impossible for the oil to be the cause of electrical failure of a correctly working, unmodified unit. In addition, most oils are mostly non-reactive, that is why they are used for frying applications and can be used repeatedly.  Some exotic oils may be a bit more reactive, but are not used for deep frying.  The fryer units are made with the expectation they will have oil in them most of the time.  Letting a unit sit too long may result in spoiled or contaminated oil, but not issues with the unit.  I would respectfully suggest that your bother is mistaken and attempting to cast blame where it does not belong.  I say respectfully as the actual words that come to my mind are more harsh.