I would think it would depend on the horse. But even a trustworthy young horse can have its moments, as our two crosses have had. I would just check out the horse, see its personality, if it's going to test you a lot, etc. If he is taking weekly lessons then he's willing to learn, and if you're helping him out and you get to the know the horse better in the beginning, he would have to listen to your advice on the horse once he starts riding it.
Have you seen the horse yet? I'd like to hear about it and see some pictures :)