She could start by talking to the head teller or branch manager at the bank where she made the deposit. She could also notify the senior vice president of the auditing department for the bank (call the main number and ask for the auditing department) to let them know that this situation is not being handled in a timely manner.
They can let her know who she should be talking to, and will probably make it a point to put some pressure on to get things resolved. If that doesn't work, call the Bank president's office or the in-house legal department.
Keep notes of each attempt, and the person talked to, and the dates. Keep the phone bills to prove that the calls were made. Might even want to record her side of the conversation at least. Depending on where you live, it may be illegal to tape the other person without informing them.
I worked many years at a bank as a trouble shooter to get other banks to correct their errors on my bank’s accounts.
I am aware of a situtation where a woman spent the extra money after a long period of time, and went to jail. She knew the deposit was a mistake, and it wasn’t her money-end of story.
I also had a situation where another bank during a computer conversion sent our bank $1 million dollars in error. Many calls were made, but the situation remained uncorrected for over a year, when it was referred to me. Their auditing department was the one that got it cleared up.