Customer goes to another bank's ATM and then claims she didn't get the money. Valid error resolution claim or not?

It would be considered an "incorrect electronic funds transfer to or from a consumer's account." The bank has an obligation to investigate the claim. This investigation would most likely involve filing a claim through your ATM network processor on behalf of your cardholder. The other bank would then check their machine to see if it was out of balance for that day. You cannot require that your customer contact the other bank first as a condition of filing the claim.

If you don't complete the investigation in 10 business days, provisionally credit your customer the amount of the disputed withdrawal, and complete the investigation 45 calander days after your customer makes the claim. Also, if you determine that an error did occur, make sure you credit your customer any fees or surcharges were assessed to the customer for using another institution's ATM.

Finally, if the other bank supplies documentation that you customer received the cash, you may deny the claim.

You can often answer your question if you replace "The customer" with "My mother..."