Is it feasible to install Sierra atop a Mountain Lion that won’t boot?
While your friend's MacBook Pro is compatible with macOS Sierra (MBP Mid-2010 or newer) I suggest to fix the issue first.

It sounds as if the HDD is corrupted. Try replacing the HDD or to boot from an external HDD (USB or FireWire).

Make sure everything runs smoothly before upgrading to a major version of OS X / macOS.