Friend of mine doesn't use a computer because he has a tremor which limits control of his hands. I've been looking around for software which he might find helpful, and found only steadymouse which works only on Windows. I would have trouble supporting him on Windows since I use it rarely any more. Possibly the program would work under wine or some other patch, but that too would be challenging (and the license is quite restrictive).

So, before I try it, I am asking here whether anyone is aware of any linux program which might be helpful for this.

As someone who came near to having that problem, I sympathize.

One solution is to get a low-res mouse set up, (large hand movement = small screen movement), and then to place the mouse on a large flat area, resting the heel of the hand, and forearm on the desk. If that doesn't enable him, get him something he can talk to. Android have 'hello Google' built in which can be used to make phone calls, etc. IOS devices have Siri. I'm an Android guy, nut having tried out google, I'd recommend siri.

Does the tremor complicate the usage of keyboard as well? If not, I would recommend enabling pointerkeys.

A pointer device with separated moving and clicking actions (for example, a touchpad with dedicated physical buttons) would be better than mouse too, but would not compensate shaking like SteadyMouse does.

I might be inclined towards eye control - I found something on github for using a "Pilot". Mightn't be much good if it needs calibrating a lot though.
Found it again - here. Never needed it, no idea of the hardware, but might be another option.

perhaps a trackball and adjusting the movements/sensitivity to very slow...

Thanks all for these (quick!) suggestions (and any more which might appear subsequently). I shall have to sit with him and discuss the specifics of how he can and cannot use a keyboard and mouse.

Wonder if the distro allows two speeds for mouse? Some have a fast speed to zoom across the screen while the slower speed allows the user to zero in on the spot.

The author of Knoppix made a distro for folks that may have some ideas in there too.

There is a sort of dynamic thing built in to most drivers these days. Faster movement = larger move steps. You can try this out by fixing a left/right movement and trying it.

A quick search online turned up a 'HeadMouse' device that "translates natural movements of a user's head into directly proportional mouse pointer movement – move your head and the mouse pointer follows along."

Another option might be to consider using a foot-controlled mouse if suitable perhaps.

Actually, memory stirs. Windows has/had some little used feature - a button near right-Alt enabled the pointer keys as mouse directions; It has this funny symbol with an arrow. Hold that, and an arrow, and your mouse pointer moves that way.

Surely, you could do that even with the DTs (Delerium Tremens). In Ireland we're experts on that subject :-). As Spike Milligan put it: "Many men die of a thirst - The Irish were BORN with one."

You can set up the numeric keypad to control the pointer. The configuration may be done via "assistive technologies" or "keyboard configuration" — it depends on the distro and the desktop. I seem to remember that clicking is done with / * 5.

simple solutions often work best - i think moving the pointer with the arrow keys is the best solution.
but of course i don't know this first hand (oops, no pun intended).
let's see what op has to say when they come back.

We had a talk about what he wants to do and what kind of support he needs. The conclusion is that he doesn't want internet in his house for family reasons. There may be factors I'm unaware of, but he seems pretty certain, at least for now. Thanks again for the suggestions; I was looking forward to trying some of them.