Do absolutely nothing. Do not touch the account, do not withdraw nor deposit funds to the account. Ten working days after that deposit has been made, if the money is still in her account, the money is hers. That is black-letter law.
Black letter law your ass. Give us some legal citations please. And your law degree and credentials dealing with these issues.
Forgive my profanity but giving someone such advice sight unseen is extremely irresponsible.
As to the poster’s concern about IRS issues, there will be an ample paper trail to demonstrate to the IRS she didn’t have this large deposit, and it was a bank accounting error. IF it is ever reported to anyone.