Yes, sometimes an "idiot light" will malfunction and give a false indication. Of course it can also malfuncion the opposite way and give no indication when there is zero or low oil pressure.
I would not take the owners word for this. Have the coach checked by a qualified diesel mechanic. You can offer this at your expense, but if the pressure is indeed low, I'd pass on it. If it is a malfunctioning light, have the owner make the repair and reimburse you or drop the price to allow for this.
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