Is my friend able to sue?

A few years ago my friend opened a joint savings account with her at the time partner. They are no longer together and do not speak. The savings account remained opened, but neither was using it. A few days ago my friend decided to transfer an amount to the savings account to put the money to the side for saving. Within a couple of hours the ex-partner took the money out of the account. When asked about it by my friend, the ex-partner admitted through text message that it was a mistake and that she would put the money back into the account. Now the ex-partner is saying that she's not putting the money back because she doesn't have to considering her name is still on the account. Is my friend able to sue for her money back considering the ex-partner admitted that it was not her money and that she'd put it back?