Philcycle, I've been gone a few days, and just saw this post. I own two TerraTrikes, a Tour and a Cruiser, but I'm confused about exactly what you mean when you say that the leg height for the standard boom is too high. I'd like to offer some help, but I don't understand what you need to overcome. Could you explain the extact problem a little more?
BlazingPedals, I agree completely with your comment. I have gone to shorter cranks on my Cruiser, but that really will not help that much. If that is indeed the problem for Philcycle's friend, he might need to go to a delta instead of a tadpole.
Actually, it was technically the bottom bracket height from the ground. He has cerebral palsy & was having dificulty with leg extension & the angle. He ended up buying a Catrike Villager, which works great & he loves it. First time he's ever been able to ride a bike at all. Thanks for the replies!