HI Lexvo
Sorry to hear about your install issues.
My guess is that Kappy is correct that you are having issues with the download itself and this is corrupting your El Capitan download.
You also state that the download from the App Store is taking several hours, this also indicates a problem somewhere in the download process, very slow internet.
Are you experiencing slow internet in general? Is this normal? Are you on Wifi network or directly plugged into a cable modem? You may need to reset your modem if directly plugged in. You may need to do this anyway if Wifi is still slow after resetting Wifi. If on DSL you are on your own.
If using Wifi…
Try turning off wifi, wait a minute and then turn back on. You can also go into System Preferences>Network>Advanced>TCP/IP>Renew DCHP Lease. Do this after the above. This will re-establish the communication link with your wifi router, if appropriate.
Do you now have faster internet speeds? Try downloading again if you do. (There are lots of speed tests out there to check internet.)
If not you need to gain access to a fast internet connection or go to the Apple Store. If a store is near, take a USB thumb drive in and ask nicely for help.
You don't say if your father's computer is portable like a laptop, if so then you can take it to a fast internet location to download the files.
Here is another way, by creating a Bootable installer. I create and keep a bootable installer for every OS that comes out.
Purchase a USB3 8GB or larger thumb drive. Insert it in any late Mac and using Disk Utility, erase the drive and format to OS X Extended (Journaled) (GUID Partition Map) (A USB 3 device is faster than USB 2 but will work in either a USB 2 or 3 port)
Now you need to have access to fast internet and a Mac with App Store account. You'll need the Apple ID and password.
You need to download El Capitan, but DO NOT INSTALL
When the Installer launches, Quit the Installer and this will preserve the download file. If you allow it to install, the file is automatically deleted, not to mention you just installed EC on the connected computer.
You can follow these instructions but one thing left out if using Terminal is that you will need to type in your password when using sudo (Super User DO) You will not see what you type so type carefully then press enter. The enter (return key) is how to execute each command in Terminal. If you use Disk Maker you will also have to use Admin/password.
Here are some links…
Create a bootable installer for OS X - Apple Support
Leaves out some steps like above
http://www.macworld.com/article/2981585/operating-systems/how-to-make-a-bootable -os-x-10-11-el-capitan-installer-drive.h…
Better for step by step and gives you info on Disk Maker
http://diskmakerx.com/diskmaker-x-5-x-est-enfin-disponible/?lang=en
Direct link to Disk Maker if you would like to use a GUI interface.
Ok once you have created a USB installer, you can now install EC on any eligible Mac without having to download each time.
PLEASE Make Sure You Have Back Ups of your computer before upgrading!!!!!
Good luck and happy computing:)
Hope this helps, Greg