Best wishes for an expedient recovery from your procedure.

You haven't specified what you are trying to do--cancel the ticket altogether or change it to an earlier/later date?

Regardless, check your fare rules (on your receipt) to see what they specify. Being that these were disclosed to you at the time of purchase, NW has explicitly disclosed the change fees, ability to be cancelled, ability to be refunded, etc.

Also, look at it from NW's perspective: nothing has happened to your mother that would impact her ability to travel on the purchased dates. Whatever happens to change her travel plans is a moot point to them.