That’s not bad.
My neighbor is in her late 80’s and tends to exaggerate so maybe that’s the case with her sister’s heating bill.
She’s a nice old lady. She said Obama “...needs to go to Dallas and keep the top down”.
I don’t know whether 600 is outrageous or not. A lot depends on where she lives, the utility (gas can be more expensive than electric for example) or the size and age of the house (older houses usually aren’t insulated, have inefficient furnaces and windows). Perhaps the lady should have question the bill with her utility company, maybe have a representative come out to test for some problem with the line or something. The rep could look over the house and make suggestions on how to save on heating bills.
My house was built in 1961. I have a new energy efficient furnace and relatively new energy efficient windows. Lowered the gas bill considerably.