I don't buy that argument. The definition of theft in the UK is to take property belonging to somebody else with the intention to permanently deprive them of it.
Taking a bit more overdraft than you're allowed, not believing your luck and getting an extra few nights out is not theft. The bank screwed up, tough titty.
Taking millions of dollars of money that's not yours, systematically over a long period of time, because someone else has obviously screwed up, knowing that you'll never be able to pay it back is another matter entirely.
There's a point at which any reasonable adult has got to say to themselves, there will be consequences. Sure, the bank offered her a loan. But we're all adults, and we know what loan means. Loan means: Repayment. With interest.
If she successfully argues that she can't count. Or was too stupid to be able to manage her money, or too stupid to realise that it wasn't her cash allowing her to literally spend millions - then fair do. I just spent the cash until the card was declined, that's how I manage my finances - could be a valid defence. Execpt when you start taking out thousands of pounds in cash, or buying Gucci handbags. Then you surely know what you're doing.
My cash card only allows me to take out £500 a day. So to get a million in cash out of my bank, I'd have to daily go to the cash machine, and take out the full whack, for 200 days! That would be, by any reasonable definition, a calculated act.
The bank fucked up. No sympathy. They put huge temptation in her way. But she took it. At some point, giving in to that temptation was a choice.
If I find £1,000 in cash somewhere, that money isn't mine. Keeping a fiver you find is fair enough, it's impossible to trace. Good luck to you. If you find £1,000 though, someone's going to seriously miss that.