Without a dock, you'll only have pre-recorded content, but ...
If your car stereo has an aux port:
6-Ft. Shielded Cable, 1/8" Stereo Plug to 1/8" Stereo Plug - RadioShack.com
If not and you have a tape deck:
BelkinÂ® Mobile Cassette Adapter for iPodÂ® - RadioShack.com
He did say that he wanted to use it as an mp3 player, so I assume he knew that...
However, couldn't he use the car kit without installing it? Use the lighter attachment to power the dock and connect the audio cable to the aux-in. The only thing that wouldn't be used is the antenna. But the Stilleto could still be used to play recorded material without having to purchase additional parts.