All possible. Indeed, as flameport has said, it's not totally impossible that the tariff she's on genuinely needs the two meters and three readings - but I very much doubt that.I'm thinking it's the remnants of a very old dual tariff supply that has since been converted to single tariff, but the dual meter arrangement has persisted. Whatever the case, it needs to be investigated, if only to eliminate the (admittedly remote) possibility that she might be unknowingly paying someone else's electricity bill.
Whatever, as I've said, I'm not sure that discussion/speculation about the meters is going to be all that useful. The situation is, at the least, confused/confusing, and would be even to someone who is not an "old granny" - so I do think that the need is to attempt to have a conversation with someone sensible at the supplier, ideally with one armed with at least some information about the history of payments and meter readings.
Basically, unless (as flameport has said) the hike in DD is to recover debts on the account, then any sane person ought to understand and agree that an electricity charge of £485 per month for an elderly woman who "..is on her own and basically stays in the lounge" is clearly ridiculous, no matter what the reason that has happened.
Kind Regards, John