Is he having the whole thing milled? If not, just split a stave out, rough it to near bow dimensions and make it less than 5/8" thick. Leave the tips and handle full width while it dries.
If he is having it all milled, I'd defer to the judgment of the sawyer on how to best do it. Generally, the thinner the better for fast drying. I'd ask if he can have a plane sawn board milled to 5/8" from the straightest, cleanest portion of the tension side of the tree. Make it 2" wide and 72" long.