You could have a bad ground.
The 12v for 02 sensor is under the passenger rug.. You want to check for 12v at the bu/gn wire and battery neg.
If you have 12v using the bat neg, then try the same using the brown wire .. if nothing there , you have a bad ground,
.. which could fool one into thinking no power to heater circuit........

Also when surges come , twist fuses 6 and 7 and tap the OVP relay.. see if that straightens it out any.

You also want to ask if he took an amp reading of the 02 sensor heater.. the specs on the heater are 1.2-2.2 amps........... if the heater is shorted and the draw is more , then the ECU may be cutting power to the heater by default..............
Direct heater 12v power testing w/amp meter in series will tell him this.
He may have already done these test, but you do not mention any Resistance values , ect. , so hard to assume anything.