What format are the letters in now? Are they still on paper, or are they in Word documents, or text files, or PDF files, or images (TIFF, JPEG etc.) from a scanner?
How does your friend want to be able to access them? Search by a fixed number of data items such as sender, addressee, subject, or assigned keywords? Full-text search of the messages?
What are your current programming skills? If you're not a good programmer already, I think you should base your solution on an existing database progam such as Access. If you have programming experience, you could consider writing a "front end" for a database engine such as MySQL. Writing something like this completely from scratch in C or C++ or whatever is for masochists only, IMHO.