Windows 8 user forgot password

Can you go into safe mode as another admin user?  If yes, then you can change a user password from there.  Otherwise, you may have to buy a USB CD/DVD portable player and boot using the CD/DVD using a password recovery tool.

Also check, 7 Free Windows Password Recovery Tools: http://pcsupport.about.com/od/toolsofthetrade/tp/passrecovery.htm

When/if you get a portable optical drive to boot from use this utility to reset the password.

https://4sysops.com/archives/reset-windows-8-1-password-with-the-free-tool-ntpasswd/

If you can't get that to work, as long as the drive isn't encrypted, you can slave the drive off of another computer, copy out the user's data, and perform a wipe and rebuild.

Not ideal, but it will get the job done.

What you may also want to do once you get this resolved, it to create a secondary admin account to where only you know the password.  That way as was suggested above, you can log in and reset the forgotten password.

I do this for my home-based customers and it has worked very well.

The yoga is an instantgo-device which means, it allows uefi booting only.
The external usb is your best bet. Use a setup dvd, do the old utilman trick and you have it back.