Try not to tell her it's stupid because that's only going to put her right on the defense. Try to go about it in a very passive way. Talk to her about what is involved with having a horse and what it takes to train and things that can happen if you don't know what you are doing (she could get hurt, the horse could get hurt).
Maybe offer to have her tag along with you and show her what it means to take care of a horse.
Trying to educate her is going to be better than making accusations and pointing out how wrong she is. Granted, she's not heading the right direction, but I think you're going to get further with her (if you want to) by trying to teach her in a non confrontation way.
Maybe Wild_Spot can help as well.
Wanted to add: Are you guys friends or is she just someone you know?