Is there a way to change double-click to single-click?

There's column view.

You can adjust the Double-Click speed in the Keyboard & Mouse System preferences.

Your father can single click on a file or folder, and then use Command-O to open the file or folder.

You father can also use Command-Down_Arrow to enter a selected Folder, and Command-Up_Arrow to go back to the parent folder.

I know these are not what you asked, but it may be a workable alternative for your father.

Message was edited by: BobHarris

The lack of single click mode this is the most glaring, problem that the mac has, it globally affects everything you do. Macs are great mostly because they are "dumbed down" but lets face it there are some qualities that are missing because the OS development has not been any comparison to windows. Macs have only had a two button mouse for a couple of years! If you are a light user of email, photos, etc (most of us) you will say whats all the complaining about? ..but if you use your mac for real work & know the advantages of single click mode, it is a nightmare. A person has to know ALL the adavantages to single click mode to appreciate what I am saying.

If he has trouble double-clicking, he could also right-click (or CTRL-Click) then choose Open. Alternatively, you could buy a 3rd-party mouse that supports button programming. For instance a scroll-wheel on a Microsoft mouse could perform a double-click when pressed.