I'm not saying to get on the horse and ride it at a walk, trot, canter and jump her for a few hours a day. Sitting on the horses back and having someone lead her around (as long as the horse is healthy) will not harm the horse.
As far as it being a mini, as long as the rider is proper weight and height it will do nothing to the horse either. We don't know if she was talking about riding or walking.
No I feel a yearling mini is just too small to ride by anyone.