If you want the money to be lost instantly, as in it's in her bank account one minute and out the next, fraud would do that. Maybe fraudsters got her online banking details, logged in and emptied her account. This could be as a result of a phishing/vishing/smishing scam - this is where someone phones/emails/text messages pretending to be from your bank, saying there's been a security breach (or some other cover story) and you have to go to this internet page and log on to your online bank, or give them all your passwords/security info/numbers/etc over the phone. They then use this to log in to your online banking.
Data theft (e.g. stealing your old mail from your bin) may result in the above if they have enough info to get through all the security questions by phoning your bank pretending to be you. (Always shred anything that has your personal info before throwing it in the bin.) There are lots of other ways thieves can steal your data. I'm currently really pissed off with Curry's/PC world as they had a data breach where customer data was accessed... no fraud has happened as far as they can tell, but data thieves can steal data from one company then use it to pretend to be you when contacting another company.
Also, some thieves put devices on ATMs that read the data on your card and video you putting your PIN number in. They then make a copy of your card and use it to access your money - you won't lose so much money instantly though, as there's a limit to how much you can take out at an ATM. They may be able to make online purchases although if they don't have the 3-digit security number on the back, they may not be able to make large purchases.
On a more serious level - the bank would be in very deep shit if this happened, and you'd get your money back eventually, but there would be the time while you're waiting for the bank to sort their shit out when you have no money - data thieves could hack the bank's computers, steal customer data and use it to empty people's bank accounts. IMO I'd find this less plausible in a story as this sort of thing would be headline news and destroy a bank's reputation.
Like the whole TSB fiasco, only possibly even worse. But it could be done if you have a fictional bank name and also make reference to all the media fallout and other severely pissed off customers.
There are various other ways that scammers operate. If your bank's on the ball about fraud prevention, they should already have given you info about the various scams and how to protect yourself. It's usually also on the bank's websites. Personally, I would not continue to bank with any bank/building society/etc that wasn't up-to-date with their fraud prevention/detection/etc. Some of the consumer protection websites will also have info about this, and on how to get your money back if you've been scammed. Bear in mind that the kind of scams that involve you making outbound transactions of your own volition, you're unlikely to ever get your money back. If the bank's at fault (including failing to protect your money and data) they are obliged to return your money but that may take time.