I am an arm amputee (Vietnam) so I have only one hand to work with. I really don't use anything different than what others use. However, I do keep it simple. Only your friend will know what works for him. It is difficult for anyone not in his position to know what will work. It may very well be, what I do, would not work for him.
All that said, I have an e-Power 18650 2800mah battery. It lasts a long time. However one has to unscrew the cap off the bottom to get to the battery, put battery in charger, put fresh battery into tube, screw cap on. That may be inconvenient for him, I don't know. It may be easier to just have a sealed battery.
I just use a cartomizer on top of the battery with a drip tip. No cone, nothing fancy. I do fill my own cartomizers with liquid; pre-filled are probably easier, but I have grown accustomed to filling cartomizers. I am a very patient and independent person; so I keep it simple. But I am very confident and comfortable in learning to do things myself.
Here's something to think about. I have a friend on this board who has limited physical ability. This person uses the Volt from Smokeless Image. This person gets great performance from their products and has automatic batteries. From what I understand, and from looking at the Volt - it is a breeze with excellent results.
Good luck and I commend you not only for helping your friend, but for taking the time to understand.