Don't have Bitlocker key, and in any case, text box for entering key won't accept alpha characters from keyboard (will accept numbers).
Back to login screen - "forgot password". This asks me to log into his Microsoft account; the email address and password he gave me doesn't work, and locks me out after too many attempts.
It seems as if I'm stuck. Can anyone suggest a way through / around this so that I can remove or reset his password? Otherwise I'll end up having to reinstall Windows, which of course will lose any installed software (I can remove the HDD to get a copy of the data).
If the volume is encrypted with bitlocker and they don't have the password or recovery key (this triggered because you made a change to the firmware) then a reinstall is your only option.
You won't be able to recover any data from that drive since it's encrypted unless they have data on a second volume that is not protected with bitlocker.