You could also go through this route
the best guide I could find that describes the way mentioned by scancode and fenixproductions (with text and pictures).
Now I don't know what type of cassette player is used, its age, what type of output is available, how strong the output signal from the tape-player is and how much milli-volt the in-port of his sound card can take. It will likely be no problem, but it never hurts to check.
Added precaution, because I have seen a lot of differences in household electronics like TV's for example. Here in Paraguay you can buy TV's that have a drastically different coax connector than the same type of TV sold in Europe. Over here people don't even know about SCART connections between your satellite receiver, your TV, your DVD, your VCR, your HiFi set. It is a beautiful, tried and tested standard for connecting electronic equipment....and impossible to buy here. If so many differences exist between (electronic) standards for the area you happen to live in, it never hurts to check. And who knows, maybe the OP has an allergy for soldering irons :-)