I don't think you can do this -- there's no available ROM I've seen that's been tested to work on the Radar 4G. I just got the same phone, and I find Windows Phone is much, much smoother and faster than the previous three Android phones I had (Nexus S 4G, LG G2x, and Samsung Galaxy S), and it does everything I need it to.
It's a really, really solid phone. Tell your girl to give Windows Phone a shot. She may end up really liking it.