Ok, I am sorry I had not read this thread sooner.
This is a tough issue due to the health aspect. In my small perspective on the health aspects with people who shake like that have others who assist them so I have no real experience with this. I shake myself but its is controllable and not all the time to largely be an issue for me like your friend.
I recommend the Zephyr Ion or the herbailair vaporizers. Now the Zephyr is not yet out with its whip for the Ion, we have been waiting on that to the point I do not genuinely believe it will ever happen but if you get tubing (that would fit the SSV or DBV size) it will fit nicely over the nipple of the Zephyr and your friend can simply press the fan button and take a draw right from the tube.
Some people use a regular whip/wand placed over the nipple and I did this with DBV wands but a little bit of vapor escapes the bottom of the glass DBV wand and this irritated to me a bit much so I removed the wand and only use the tubing. No leaks at all but heat will bend the tubing and you have to hold it up to keep it from pinching.
The Zephyr has a large bowl size that is wide so it would be easy for your friend to load and is very efficient and even in my view. Using bags are easy and simplistic but likely not realistic for your friend to change them at the mouthpiece. Bags attached to the mouthpiece would be no problem to use as you simply place it on the nipple and press the air flow button. The buttons are spaced well and the unit is very durable with a large easy to read LCD screen.
It has been quite some time since I used a herbalaire and I believe this is also a good option for your friend but I recommend looking in a herbalaire thread for more information. The herbalair is also very durable but is said to be better at bags than using whip. Price wise the herbalair is a great buy now I understand.
If money is an issue, i would consider the herbalaire first then the Zephyr second but if user friendly is paramount than I would recommend the Zephyr due to it being easier to use/operate for a person who shakes in my opinion.
I would not recommend a log vape nor desktop vape like the DBV or SSV due to the glass and need to stir. Not so much stirring needed with the DBV due to wider air path but it still did need it. I love my DBV but I have to tell you, when I would start shaking it is scary when you drop the wand, its glass and has happened to me more than few times. Also, depending on how bad your friend shakes, it may be potential that if he is shaking and turns the vape dial to on or to adjust the temp it can be done with too much force and is possible to break the dial. Rare, but a possible scenario I think.
Regarding the all day thing, I would forget that and look for what is realistically easy for people to load and unload. Unless they hit very small amounts no vaporizer will fit that bill I think.