You could certainly do that if you want to but I don't believe apple offers downloads for any of their preinstalled apps aside from iTunes, Safari, Quicktime and maybe one or two others.
If you're having problems with Calendar it would probably be best to try and just fix the problem, lots of times simply deleting an apps plist files can fix a problem.
If you really can't fix it I think your only way to reinstall a stock app without getting a copy from someone else or your own backup from before you had problems is to do an OS reinstall.
If he has a time machine backup that is from the time before the calendar app started to act up he could restore just the calendar app from that. As mentioned first delete the plist and see if that fixes it.
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If you're having problems with Calendar it would probably be best to try and just fix the problem, lots of times simply deleting an apps plist files can fix a problem.
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The plist suggestion is fine. I wouldn't recommend deleting any stock app without knowing what the issue is and all the details for fully restoring it.
Create a second user account (which is a good idea to have around anyway) and try starting Calendar from that. If it starts then there is nothing wrong with Calendar.app -- it's either the plist file or the Calendar database (stored in ~/Library/Calendar).
I created a second user account and after that Calendar did start. Next I removed info.plist from /users/calendars. The icon jumps now only one time, but Calendar did not open.
I removed the entire map from /./calendars, but it did not resolve my problem.
I really don't know what to do else.
Next I removed info.plist from /users/calendars. The icon jumps now only one time, but Calendar did not open.
I removed the entire map from /./calendars, but it did not resolve my problem.
I really don't know what to do else.
You've lost me here. I know of no "info.plist" and there is no /users/calendars unless you have a user called calendars. And "/./calendars" makes no sense unless you are saying you deleted ~/Library/Calendars which is pretty drastic as that will delete all your calendar entries.
It looks like the plist files are ~/Library/Preferences/com.apple.iCal.*
They still use the old name before the application name was changed. The command:
rm ~/Library/Preferences/com.apple.iCal.*
will get rid of them all. Getting rid of plist files is relatively safe since they are regenerated if missing and you will just lose any preferences you have set.