Maybe might be best to slowly start introducing him to the outside world on foot? If he is fine in an arena then yeah, maybe that's all he knows but I would still be a little worried that even in an arena with a novice on him he could still spook.
Definitely age doesnt always mean a better temperament especially if the horse hasnt been exposed to much but I would still be looking for someone older for kids. Has your friend ridden him in an arena yet and if so how was he?
People do it all the time when selling horses - not being 100% honest to make a quick buck. But possibly your friend fell in love with him and followed her heart instead of her head which happens all the time, I have done it too!