Create bootable EFI system partition from macOS Installer's firmware files (.scap, .fd)
macOS will boot fine without an EFI partition. Unlike most operating systems, macOS stores its boot files inside the OS partition. Apple computers ship with firmware that can read HFS+ partitions (2016 and later models can read APFS volumes as well by default), so they can read the boot.efi file from the partition. When you upgrade your computer to High Sierra, the firmware gets upgraded as well, getting the ability to boot from APFS partitions.

Your friend probably did more than just overwriting the EFI partition. You will probably need to backup your data and reinstall macOS.