A friend with a Panasonic microwave, Inverter model, physcially a big
one, says it is tripping the circuit breaker after about 5 minutes of
running.

They've reset the breaker several times and it's the same each time.

What's your best guess?

I havent' looked at it yet, but my best guess from afar is that some
of the primary windings in the transformer have shorted, and thus the
impedance of the primary has decreased and it's drawing more current
than it should, more than 15 amps. Does this sound reasonable? If
not reasonable, then possible?