iMac remotely firmware locked

2015: Bought my mom a 21.5" imac. She passed away and I didn't use her iMac for over two years (no updates).

2018: Now I'm trying to fire it up and her password doesn't work. I get the error message "Your computer is disabled. Try again in xx minutes". I've restarted in recovery mode and tried her Apple ID and other assorted passwords to no effect.

How do I get this iMac unlocked and working again?