You are probably, at minimum, going to need the installation media, in order to boot your machine.
That should allow you go use the "Repair your PC" link on the second page after the language and keyboard screen to access the Troubleshooting -> Advanced Troubleshooting screen, from which you might try a System Restore to a point in time, prior to the problem and changes that he may have made.
That would probably be my first step.
Failing that you might be able to rescue it by following this article
http://pureinfotech.com/how-reset-windows-10-forgotten-password-regain-access-computer/
It involves, again, using the installation media to boot the computer and using Shift + F10 to acces the command prompt.
Follow along the step by step to activate the hidden administrator account.
From there you might be able to rescue you other user account(s).
It works. I know, because I've used it successfully. It explains how to do it and then how to revert the changes you make afterwards.
Failing that you're going to need something like
http://www.iseepassword.com/reset-windows-10-password.html
I personally purchased the Enterprise version a while back.
Best $50 bucks I ever spent.
Good luck.
Regards
Randy