> we told her to check with her neighbors and the complex management

Excellent advice, was going to suggest the same. This will give her the best insight as to if it's normal or out of whack.

So the bill may be "correct", insofar as the meter reading was accurate (let's assume it was), but there may be other things happening here. If it were me, I'd want to ensure that the trend over time is consistent--so, did the apartment in the winter months last year and the year prior have similar bills? If it did, then we know that nothing is suddenly awry here and it may be a matter of drafts, leaks, windows, doors, etc. needing some sealing/insulation attention.

If the $400 just doesn't line up though, you should put on the detective hat and start investigating and eliminating possibilities. Is a neighbor using her electricity (either intentionally or not--like recent electrical work and a circuit was wired wrong and is pulling from her apt's meter but feeding another apt.)?

Is something else jacking the usage up? Space heater? Significantly higher usage of an appliance like the oven or dryer? The fridge old and constantly running?