Your friend's problems are largely about control and typing. They may be able to use one arm, but it may tire fast. Consider alternative mice, including trackballs. A keyboard if possible, perhaps a tablet with keyguards. If not, some kind of switch control for an onscreen keyboard: it will be slow but maybe more useful. One Switch Mouse, above. Built-in features, like Windows here: https://support.microsoft.com/en-gb/help/27936/windows-10-ma...
Eyegaze: cameras that track where you are looking. Hard to use, probably not best if your friend can use an arm.
Speech recognition: you really want Dragon Professional on Windows. If you can't get a trial of that, macOS speech recognition. You MUST MUST MUST have a decent mic. Windows 10 machine, try Windows Key + H in an edit box, or this freeware (my employer again): https://www.clarosoftware.com/login/admin/labs/#clarodrake
Expert in California: https://store.rjcooper.com/
Your friend is still regaining function, which changes the dynamic: using the built-in features, maybe with a trackball, might be best until you know where they end up. Again, this is an OT call.
Good luck!