(2) Brush-up on keyboard navigation. Most desktop applications are good in this respect but many web pages are in the stone age.
(3) Tune the driver parameters.
(4) If the user has particular issues (which may not all be motor related) then focus on a 'way to do it'. For example a positive one-click even if the mouse button takes a hammering.
(5) There used to be special drivers but 5 years ago when I looked they seemed to be dying-out.
One of the issues is losing faith/confidence in one's own skills and getting more nervous/flustered. Try and find a fun and 100% no-problem' way of coaching them. Another issue with poke 'n hope is things go wrong and much more confusing. For example a double click or a right-click instead of a left click will start weird dialogs or actions. "Hey! I wandered over that email address and now it's asking me lots of questions!!" and so on. So it's up to your patience to reduce the stress. (And if you're trying to sort it out by phone then without something like Teamviewer you're going to get in a muddle and the user is going to feel a time-wasting idiot and failure.