How does one copy cassette tapes to computer?

All sorts of computer audio cards have an analogue input.
All one would do is plug the cassette deck into the analogue inputs of an audio card with such inputs.

With this; you can input the casette deck's signal, via your computer's stereo mic input and a stereo 1/8" to dual RCA adapter. You can then save the files to a wide variety of formats, at various sample rates, of your choosing(ie: Hi-Rez MP3):(http://download.cnet.com/Mixcraft/3000-2170_4-10285311.html) There's a free sample download available.

I can't help with recommendations for doing that on a PC but on a Mac the combination of the 1/8" to dual RCA adapter to the built-in audio input and the "CD Spin Doctor" application that comes as part of the Toast software package works well.

He'll get one complete audio file per cassette side. If he wants to separate those files into individual tracks it can be done manually with any number of audio editing applications. Audacity is a good free one for both Mac and PC platforms. There is a function in CD Spin Doctor to automatically separate the tracks but it almost always requires some manual tweaking. I found it was too time consuming to separate the tracks so in most cases I just play the entire side, just as you would do on the cassette player.