A friend of mine is running Ubuntu and got
GRUB RESCUE. Can they use Mint ISO to repair their Grub? As I don't have Ubuntu ISO?
If the Ubuntu installation is still present (and only GRUB was lost), sure, you can use any distro that has live booting to do so.
If all you need to do is install grub, and updating isn't necessary, then you don't need the
If you have a separate boot partition, remember to mount it as well, after mounting
Since grub is standard boot loader, you can use any distro that uses grub as its bootloader.