Can customers return just one item from an order with several products?

Yes, if you allow it. On Plugins → Return Settings, open the tab for that action (for example Refund), tick Allow partial Refund and click Save Refund.

The customer then sees an Items list when requesting a return. They can pick specific items and quantities, or leave all unselected to return the whole order. Without this setting, every request covers the whole order. Partial selection is set separately for Refund, Exchange and Cancel.