Never download and install something that you just happen to find somewhere !
Do you have a good virus scanner. Did you download things from unknown origin before ? Perhaps you should wipe your drive and reinstall Windows.
It depends on the Mega 2560 board.
What harm could it do to install the Arduino IDE ?
Is the other person using Windows 7 ? That is weird. I hope that computer is not connected to internet.
The official Arduino board has drivers that are included with the Arduino IDE. When you download the .zip file from the **official* Arduino site: https://www.arduino.cc/en/software, then there is a folder "drivers", that contains all the latest drivers for all the Arduino boards. I suggest to install the whole package for all Arduino boards.
A clone might need the CH340G driver. If you need that driver, then download it from the official manufacturer's page: www.wch.cn/download/ch341ser_exe.html.
There is an other option: Tell the other person to use linux and add himself/herself to the "dialout" group. That's all for linux. The drivers are already included.