Well i heard that apples have a large amount of sugar and are not so great for horses. And that green apples are not supposed to be givin to them at all, because they are more tangy and can mess with thier digestive tract. I have a walker who foundered about 3 years ago, and the vet said that apples were the worst thing for him because they have so much sugar, the vet suggested not to feed him anything else but his grain(which is rice brand) and his hay.
Anyways, i am sure all the apples and carrots all together like that is really bad and could cause a potiental problem.
Well to tell her, i would just kinda get on the topic of horse feed and treats. Then i would mention about you hearing that apples werent so great for horses and they tend to cause alot of problems when feed in bulk. Maybe then she would get the hint, without you having to come straight out and saying it.