If he finds that sender wire laying on a chassis ground he may as well get a meter and check for continuity through the gauge like a pulse voltage coming though it.
Reason being, the oil gauge can and does peg out and die without being noticed.
I had a part project going ( DeMon Centurion ) that would provide warning lamps and/or dingers for these type of fault conditions.
Its installation, adding the LEDs in the gauge screens, etc.., proved far too complicated for the average owner and it really wouldn't benefit the average owner .. so I scraped it.