Oh my lord, I thought it was bad being that with 9 of us in a 5 bed house that eats electric, but to hear your mum pays that for just her each month - if I didn't have a dodgy heart before, I certainly do now!!!
I'd definitely get that checked out, maybe the wires are crossed and she's paying her streets bill's, too!!
I know electric radiators can be expensive to run, have you considered oil radiators for her? They might be cheaper.
My mum used to boil her kettle in the morning then make her first cuppa and put the rest in a flask or 2, so she didn't have to reboil all the time!
Your mum doesn't use a washing machine or dryer at home (I'm assuming you dry and iron her clothes, too) so even with today's prices I'd say that's way too high!!
I'm currently in a 2 bed house, end of terrace, radiators in every room, lights on downstairs a lot as I suffer with SAD & our lounge is shaded, we have tv running a lot with pc & phones charging. More so when the family are here (anywhere between us 2 + upto 4 - 9 people) washing machine, no dryer, water on a lot, my daughter and children come for baths/showers as their water heater is useless, and we still only go thru about £40 per month. Possibly upto £60 in a really cold month. We're on a meter, too. Gas on meter, too, about £20 per month. I'd have a fit at your mums bill's!!!
You can buy a Smart Meter that plugs in and tells you what is using the electric, I think my daughter paid around £99 for hers - I could be wrong - but it goes onto the bills so you're not forking out a lot in one go, I'd see about getting one, if I was you!!
I'd buy a body warmer, too. Lol