I have had Sirius for about a year or two now and LOVE it! Depending on what kind of car she drives you have a few options.
1. If it's a GM vehicle then you can buy a unit that goes behind the dash which will allow her to use her stock radio to control and listen to the satellite radio. Very clean looking and works perfectly - I have one in my GMC truck.
2. If she has a different type of vehicle or an aftermarket radio that doesn't already support then she could get what they call a "dock and play" radio. The Sportster 5 radio is very nice and has some nice features like build in memory to allow her to pause, rewind, etc live stations.
3. The last option would be their portable radio, similar to an ipod but allows you to carry the radio on you. The only one I'm aware that's available if the Stiletto 2. Has built in memory and you can add a memory card for extended space. Will play MP3s, Internet radio and of course satellite radio.
The nice thing about this radio is that it's like a "dock and play" radio above but more portable as you can carry it on you and listen to the radio. So most all vehicle docks, boom box docks, home theater docks will work with this radio. I also have one myself and use it while I'm at work on a dock hooked up with computer speakers - works perfectly.
If she's just looking to listen to it while she's in the car I'd go with option 1 or replace the factory radio with an aftermarket radio that supports satellite radio. However if you want more options in terms of where you listen to it then I'd go option 2 or 3, Option 2 if she doesn't plan on listening to it on the go, say out on walks or working in the yard.
I'll tell you I love satellite radio, in the year or two I've had it I have NEVER listened to FM radio. Satellite is awesome, every type of music, talk, comedy, news, and sports. Personally it's worth every penny and more. I'm sure she's going to like it!
-Michael