There are several different Dahon Radius handleposts. Googeling "Dahon Radius -V Comfort Handlepost" is not giving me any impression of what exactely you are looking at.

Most of them would fit on her bike, but what you need to take a close look at is the angel at the bottom of the stem. You want one where the stem goes straight up instead of forwards away from the rider. This can be difficult to spot on a small picture, you may want to contact a seller or two to ask. I would buy a stem where the handlebar is atatched on top of the stem, not one where you use a stem extender to place the handlebar in front of the stem.

Stem and seat tube on these bikes is made to adjust to most rider sizes. They do that by angeling seat post and stem away from eacother, so that the more you lift them up the longer reac you get. You want a stem that goes as close to straight up as possible.

this bike: Dahon Mu SL - C H WHITE & SON

has a stem that is going almost straight up.

This bike:Dahon Vitesse D7HG - C H WHITE & SON

has a stem that has a more forward angle at the bottom.

The bike is made up of several components so the same stem used on two different bikes/frames can give a totally different result.

First find out what stem is already on the bike so you do not buy the same again.

I know some peopel have turned the stem around 180 degrees to fit children. Maybe this is an option on your bike- or maybe not.