Having a little experience in these matters (having worked at Eon) What I would do first is a little experimenting.
1) When practical to do so turn EVERYTHING off at sockets, switches & light sockets, after 10 mins or so the dial should stop spinning around (to prove that nothing untoward is draining away power (and that the meter isn't faulty)
2) turn the appliances back on 1 by 1 (the ones that run constantly i.e. fridge, freezer etc) to see that one of them isn't sending the dial spiraling.
3) Are any of her appliances old? by old I mean ancient and possibly faulty?
4) Has she purchased any recent new appliances (check these too they can be faulty - TV's are a prime culprit and if a faulty lcd screen can pull up to x4 the supply they should)
5) It may sound silly but none of the neighbours are tapping into her supply are they??? One customer who I visited due to having an extortionate bill, was found to have a rogue wire running up in the loft through to the neighbours address

6) Check that the meter is set up correctly ie. the meter no. on the meter eg. (S12M12345) tallys with the one shes being charged with on the bill (Known em in the past to input the incorrect meter no's on the wrong bills, so for eg. if you & your neighbour have their meters changed at the same time, some 'herbert' may put the wrong meter against the wrong property.
7) if all that fails then enquire about having a check meter installed, if it is Eon's fault they'll have to re-fund you the cost of having the meter installed + any over payment. However if its correct you have to pay for the fitting of the check meter.