> When I was a TA in grad school, I had one student who couldn't double click.
This isn't at all uncommon actually, especially among adults age 50+. Double-clicking a mouse requires a certain amount of manual dexterity, but we've all forgotten that because we developed it a long time ago and to such an extent that it's become effortless.
Recommend they get a trackball; a big, beefy beast that can be slowed to a crawl. Better for the neck, better for double-clicking, and easier for positioning the cursor (especially if they have Parkinson's).