God, I love threads like this. First, of all have you asked her??? Start slow, if she is ok with giving it a try then go from there. Don't buy an expensive bike, buy something that she likes and is comfortable on first of all. The first thing all my girlfriends ask me, is how can you sit on a small seat for so long??? I tell them that you get use to it.

I think for most women who are new to this is and worried they are concerned with COMFORT AND FUN!!!! MAKE IT FUN.
Then go from there. If she really takes a likely to it then you can always up grade the bike and etc.

GOOD LUCK AND HAVE FUN!!!