If he’s not an experienced horseman, then no. Especially with a thoroughbred. They are tall horses, and can be a bit spirited. Taking a fall from one could break his neck.
Plus whenever I am pleased to see someone mount a horse they are max 125 pounds wet.
Horses didn’t work out well for Superman so I say no. https://www.pennlive.com/entertainment/2020/05/actor-christopher-reeve-best-known-for-playing-superman-paralyzed-in-accident-25-years-ago.html
I cringed a little at the thought of a child doing that, too. Nice of her to donate them, but horses are unpredictable animals. And no, President Trump doesn’t need to ride any of her horses.