Hmm. Fair point
If you can look at his bills and find out his actual consumption from last year, I would go on the comparison sites, or at least the energy company's site, and input the kWh for an estimate of cost on his tariff.
Might also be worth double checking whether he's on the best tariff (assuming he's not in a tie-in period).
I suspect in this case that they've put the DD up in the hope he won't notice and they'll get lots more interest on his credit balance in the meantime! Ring them up and challenge the DD payment. I've successfully done that in the past.