Proof of deposits into account required else account will be frozen. Bank: First National bank.

Someone close to me have been threaten to have their account at FNB frozen. No further information was given. The person is an elderly person and had to get someone to take her to the bank. She was then told to provide proof of certain amounts that was deposited into her account. I have never heard about anything like this. I have been banking over 30 years. Can someone at FNB phone me and explain this to me? Is this even legal?