As others have said, every person is different in their abilities and limits. And I know nothing about your friend's situation, so I can only tell you about the situation I've worked with.
My aunt was born with cerebral palsy, and she has always had much better control over her feet than her hands. Her solution was to place an ordinary trackball under her desk, (the large kind, not the marble sized one) and she uses her bare foot to control it.
Because it's foot operated and she can't really clean it effectively, it gets dirty much faster than a desktop trackball, and so she ends up replacing it more often than you would a mouse. But overall it's been a pretty cheap investment, and one that works for her.