Then if it is just horses, it is all down to the horses relationships with one another. Therefore, it would be a good idea to put her out with the horses she rides with so she can assert her dominance over them there rather than when riding out.
I agree with Sharon here. Make sure she is comfortable with things being close to her through desensitising.
...Or next time she kicks, put her straight into work and repeat until she realises that every time she kicks, she will just have to work. Make the wrong thing hard and the right thing easy.
Know that horses will always attempt to kick others if they're trying to show authority and dominance. Allow horses to get used to each other in a field before riding out with them to make the chances of kicks less likely.
Do a lot of ground work and desensitizing work with her. There is a horse just like that in our barn and she kicks if another horse is in the 2 meters of her. It might take a long time for her to stop this habit so tell all the other riders that you ride with what your horse does. This way no one will get hurt if your horse does kick. I wish u luck!