What a wonderful job to pay off a substantial debt and not take the route she did.
I am sure there have been very difficult days because of the pressure to pay it off. I like the idea of writing the letter - I find this is a great way to relieve and express something without ever getting in someone else's face.
Or maybe you could write a letter to yourself about the pride you have in what you accomplished, detailing what it took to overcome this mountain. Wow, remember this huge accomplishment & if in the future you feel overwhelmed by a responsibility or something you want to accomplish, you can look back at this and know you have the stamina and backbone to see the project through to the finish line.