I was in the TNH category myself. Even if the house was a 20 bed mansion.
I’m wondering had she the timer set to go on all day without realising? Or the thermostat completely cranked up? Even at that, she’d be using it much more than an hour an evening.
She must have got mugged off by the estimated reading. No other explanation for it. But the fact the direct debit came as a shock to her is beyond naive