Why not spin it this way...
Take away her input/maintenance capabilities on your computer system. Tell her you realize she is overworked and that now someone else will be responsible for the maintenance input. She can still take the information from the customer, write it down on a "maintenance to be done" form... but your bank gives the input duties to someone more capable.
In the end, she may be relieved! I bet she is still there because she loves the interaction with "her customers". This way, you don't take that away from her.
(We have a similar situation at my bank and our customers absolutely LOVE our lady. We would not dream of firing her!)