That’s what it’s all about, isn’t it ! 
Now to dream up the split vanity top design.
Wrong cedric. Your “average” human being prefers a 36" high cabinet. ADA (Americans with Disabilities Act) law requires that all countertops (which will be used by the handicapped) must not EXCEED 34" in height. It is 2" lower than the standard.
I used to work for a millwork subcontractor, I know all about this.
If he is making it himself, then that’s just fantastic. He can make it however he is comfortable. But I know he wants to know a ballpark, since he obviously doesn’t have any countertop to test right now. Personally, I believe the naval might be a tad too high. I think rather than thinking about how high his hips are and his naval is, you should consider his hands. Those are the things going to the faucet and the the sink. And don’t forget about forward reach. It might be perfectly comfortable at the rim of the counter, but when he has to reach for the soap that’ll be another story. You should be able to lightly rest your hands comfortably about the middle of the countertop without having to stretch your arms too much. This height obviously depends on how long his arms are. Get some wood and stack it on top of an existing countertop and let him tell you which height is most comfortable when he places his hand 1 foot in from the edge of the countertop. You might want to raise it 2-6 inches to account for the sink. That IMHO is what will get you the most comfort.