If the lighting circuit happens to be on the same circuit as the burner he
should see some dimming when it starts. Not flickering though, that's an
altogether different situation. If the burner is on a dedicated circuit and
he's getting dimming on a separate lighting circuit, he's probably got a bad
connection on that leg of his service or possibly the neutral of the service

The burner is on a dedicated circuit. I am seeing dimming on a
separate lighting circuit.

Does that mean the connection is bad on the lighting circuit or on the
burner circuit?

Michael