Andrew's friend put Linux on his computer without his knowledge and wants to know if he can get his data back. Leo says it's possible that he didn't wipe the drive at all and just added Linux to it. If that's the case, then the data is probably intact. If he formatted the drive, then there's a problem.
The chatroom says there's a program called Boot-Repair which can see it and then setup the boot loader in order to fix it. There's also a boot repair command here - https://help.ubuntu.com/community/Boot-Repair.
But if he wiped the drive, it's all gone. This is why it's so important to ALWAYS backup before doing something major to a computer.