Ohh, don't worry girl! I have had plenty of cases where I thought I could trust one of my best friends, it turns out she was a back-stabbing, two-faced liar, selfish bee-ouch! You've still got your other best buddies right? Just find someone that you know you can trust. I have been best friends with a girl for more then 5 years and I know she would do anything for me and the same for her.
If they wanna get hurt, and they do, this is where you need to tell them "I told you so."
I would just write back to them and say that you were simply looking out for the two of them and didn't want them to get hurt because you care about them. Tell them that you weren't saying they were the ones for the job, just simply stating that you didn't want them to get hurt by an untamed horse. Also say, it takes many years and lots of education to properly break-in a horse.
Keep your chin up and don't let them get to you! I did and I went downhill and lost my self-esteem and happiness, don't let that happen to you.
I hope I helped, good luck