I find that when training horses, consistency is key. And to be consistent we must first be confident.
There is a horse at my barn that typically lunges/kicks at everyone except for his riders, myself and 2-3 others. The people he knows he can intimidate are the ones that he is the worst with. All that I do is look at him in the eye and treat him as if nothing is going to happen when I walk by, and if something does, I kick him in the gut, growl loudly and he stops. That's basically a life/death situation though.
The reason the horse lunges for your friend is because he probably thinks nothing of it and just expects the horse to lunge and so it does, and when it doesn't he cracks the whip at it. It's really not that difficult, if the horse knows how to lunge then we just expect him to with no questions asked. Watch your friend lunge a horse and try to mimic his body language.
Also for disciplining the horse, you really shouldn't have to do anything too drastic. It depends on the behavior, but usually I like to think of disciplining as interrupting and correcting the horse. If he is walking and we want him to stand still, then stop him and reverse him back to where we want him. If he is not picking up his foot, then we poke/tap him and repeat the aid, etc.. Basically think of it as teaching a child. We help or correct when necessary and otherwise try to interfere as little as possible.