I like that you are wanting to take your time and let him hit 3 before you start getting on him. But he's just a baby, and at that young an age, I'd work on desensitizing, picking up feet, standing still, things aren't so scary, and then of course respect for the handler. One he does those easily and consistently, I'd let him be a horse, turn him out, let him play, be with a "herd" so he learns boundaries, and get him out several times a week just to groom him, check for cuts, scratches, clean his feet etc., and be done. I wouldn't really even start ground driving until he's 2, unless you are planning on doing halter, conformation etc. type shows with him, there's no need to speed things up, though I know its hard to wait and just sit and watch.
As for your friend, if she's not going to listen, she's not going to listen, but drafts and warmbloods mature slower than the "hot" breeds, and are often started slower because of that. Even though he's a "big" horse, he's still a baby as well, and his back isn't developed enough to support a rider, even for short periods. I've seen the results of someone riding a horse too young, (yearlings,and early two year olds), and it isn't pretty. I'd just remind her that he's still a growing baby, and shouldn't be ridden, and give her a few facts about growing draft horses, (I know there are some draft people on here who can give good advice), and see what happens.