I have the Monster FM transmitter/cig ligher power adapter. It sucks and, from what I've read, it's one of the better ones. I recommended finding some way to hardwire it. Does her HU support a CD changer? If so, then there is almost certainly an adapter for the iPod.

I don't think it's worth bothering to do with an FM transmitter. But, if you must, then get one that lets you put in any station because, at least in my area, it's very hard to find an open frequency. None of the 8 that are on the Monster work very well around here.