Think of it this way: If you charge $40 a shirt, you will be making basically $20/hr (at best), and you will be SLAMMED with orders. Say, 40 shirts a week. (Because sewing time is quickly going to be way overshadowed by fabric/button/fabric store time.) That's $1600/wk. at a break-neck production pace. Your hands, arms, shoulders, and eyes will quickly tire. The quality of life you have outside of "work" will deteriorate as you spend more time in pain, and less time on social things. Or... You could charge $100 a shirt, make 3 shirts a day, and still take home the same amount of money without damaging your body, health, or social life.
I think he could definitely command $100. That said, it's all about having access to the people that would pay $100. If you do't have access, you don't have sales. I think that a $50 shirt, including shipping - which shouldn't be more than $5 to anywhere in the US, is a good start. Once you can't keep pace with those orders, raise the price to $80 and so on... You can always raise the price. No shame in that at all. But find your market first. Have people asking them "where did you get that shirt...?" and go from there. Edit: Also, when you're starting a business you almost should be so busy your hands fall off. The more shirts he's making the better he will get. Much better for him to be cranking out 6 a day than 3, if nothing else so that he can further hone his craft.