Get a
Sik Din to get a true line out. It bypasses the internal amp in the iPod and sounds a lot better than going through the headphone jack. If she has an aux input on her car stereo, get a Sik Imp instead as it has the car charger too.
I've played the line out through a cassette adapter in the car and it sounds good too, though I just yanked out that head unit and put in a stereo with an auxillary input.
FM transmitters suck.