Yes, a stallion can be mannered, become well trained, but that is not the point here.
This is a three year old colt, and unless owned by someone that is in the 'breeding ' business , is better off gelded, for everyone
Yes, youth can ride stallions, and even show them, in stock horse venues, anyway, but NOT IN YOUTH classes. They can show them open
There remains the fact, that should an injury result, with the rider loosing control of that stallion, even around in heat mares, the liability is strictly on the stallion owner/rider
Your daughter is not a kid, growing up in a horse breeding/raising operation, where a stallion , used to promote that business, is going to first be mannered and trained to the point,that an experienced youth could ride him, but he is a green horse, bought for a youth, with the added complications of hormones. Just riding a three year old green hrose, is enough of a chellenge for a fairly green rider, who is also a youth
HAVE HIM GELDED, now !
When I was about 15, and horse crazy, and had made do with the draft horses we used on the farm, to learn to ride on, my step father gave in to my begging for a saddle horse, because my non horsey mother wanted me rewarded for all my hard work in those tobacco fields
My step dad used hroses, but was not a horseman-there is a difference! He was practical , though, and since our work horses were mares, seemed very logical to then buy me a stallion. This stallion was cheap, because he proved to be spoiled.
He reared, went over backwards, tried to run me into things, bolted-you name it.
Yes, I was young, fearless and also lucky, as I could have been killed or paralyzed.
Once I grew up, started to raise horses, I did raise and show several stallions, and even let my older son, show one of them, open, when he was about 15, But by then my son had been starting horses with me, and \i had shown that stallion enough first, that he was 'broke and mannered'
There is only one reason to keep a horse a stallion, and that is if you are breeding horses, and that stallion has superior blood lines, and has proven himself, or at least is in the hands of someone able to prove that stallion. This takes either the experience, or the money to have professional train, ride and show that horse.
That does not appear to be the situation in your case, so make that call tot he vet!