This might be slightly more complicated if they are married. Even though the account is in her name, is it not the case it counts as marital property?
I do agree with the fraud aspect of it. And it's also important your mum knows she is a party to it, should it ever come to light. The fact she didn't do it isn't a defence in the eyes of the law, the fact she hasn't reported or closed the account means she is implicated in the wrongdoing.