My buddies Rotax 447 has a CHT and EGT gauges they are from EIS company.
His rear CHT Gauge is reading 450 to 500 degrees and the thermal coating around the sensor keeps melting away a bit where it touches the Spark plug.
His front one reads 180-200 and the EGT is in the green. Engine is running like a champ and has no other issues.
I told him watch EGT and and LISTEN to the engine, but that CHTs are notoriously unreliable.

One friend suggested to get a sensor from a different company, (Westach) or to switch from J or K type to the other.

What does the brain trust here say?
I flew for years with no CHT gauges and just an EGT and was quite happy.

I would suggest he first tries swapping the sensor and gauge to the other cylinder and see if the issue goes with the gauge set...

Then I would buy a new pair of matched gauges/sensors and sell the old ones...

Too risky having temp gauges on a 2 stroke (or 4 stroke) that cant be trusted/relied on.... damage can happen more or less instantly... which is not a good thing on a flying machine

Maybe he can borrow some known good gauge and sensors from a friend to confirm what his gauges are telling him?

Edit: If it is the ring under the spark plug type sensor, you can put 2 (the existing and a second to confirm) under one plug, temporarily to see if the original is reading correct. A thermal laser temp gun aimed at the plug are can also confirm if there is a huge discrepancy from what the gauge is saying.

I suggested swapping them too. They are cut to fit. He doesn't have any slack.
But the laser pyro.eters a good idea!

What Muz said. Put both sensors under the same plug and the gauges should read the same. If not, a gauge or sensor is faulty. Another possibility is an incorrect heat range plug, which could readily hole a piston.

Hey thanks man, had t even thought about that one!