So did you try to boot off the installer and get to safe mode that way?
boot from installer, on 2nd screen after languages, choose repair this PC, not install
choose troubleshoot
choose advanced
choose start up options - this may not be here which is a pain
hit the restart button
choose a safe mode (it doesn't matter which) by using number associated with it.
PC will restart and load safe mode
Does friends PC have 1 hdd or two? does he have all his data on C drive? if so, can use this to save any of it: http://www.howtogeek.com/howto/windows-vista/use-ubuntu-live-cd-to-backup-files-from-your-dead-windows-computer/
Since PC is only 2 days old fastest fix might be a reset
boot from installer, on 2nd screen after languages, choose repair this pc, not install
choose troubleshoot
choose reset this PC - both choices just effect C drive, one keeps files( library folders) and settings, and the other wipes C drive
PC restarts and re installs win 10