He once managed to solve a dual-boot problem for me. The other times, he didn't manage to fix my problem. I guess it works sometime.
I run a dual boot system with Windows 8.1 and Linux Mint. For unknown reasons it sometimes takes minutes before the grub menu appears. Boot Repair has been 100% successful in restoring my usual boot time back to 7 seconds.
This app is kind of a gamble. It used to work perfect with legacy BIOS setups, but now having some trouble with UEFI and multi-boot setups. It will either work flawlessly for you or it will funk your GRUB up. Before using this you must be using Timeshift or other backup tools regularly, and you'd better get yourself familiar with some commands (as in https://www.linux.com/training-tutorials/how-rescue-non-booting-grub-2-linux )