Three options:
1. He can use a cassette adaptor (I do this at the moment), which works OK, but is not too pretty
2. He can use an FM modulator which has to be connected into the antenna wire and installed somewhere behind the dashboard (I have bought one but am still struggling to install it)
3. He can use an FM transmitter, which may not be allowed in the UK, I tried one but it did not work very well (because the antenna is too far away).