backuphard drivepasswordrestoretime-machine
I've inherited an older Retina Macbook Pro from a deceased relative. I don't know the password for their user account, and I would like to make the machine usable.
I have a Time Machine backup from my own MBP, however when attempting to restore it, I'm asked for the disk's password: 
I also can't mount the disk using Disk Utilities, as it again asks me for a password to unlock the disk.
I don't mind losing data, I just want the machine to be usable again. What are my next steps?
In the Applications directory, there is a subdirectory called Utilities. In Utilities you'll find Migration Assistant. Or just use spotlight to launch it.
Migration Assistant has the option to restore from another machine, a Time Machine Backup, or another disk.
Make sure the TM disk is plugged in to your wife's new machine, and you should be able to select it from a list of volumes.
Absolutely - this is a good plan in general - having a bootable backup in addition to a Time Machine interval backup.
I like to get a SD card / micro SD card and install an emergency boot OS with just enough tools to fix any situation.
I would do that before making any changes. Next, you will want to measure how large your drive is - open Apple Menu - about this Mac and look in storage:
For this Mac, you would need 185 GB of space.
Best Answer
Go back to the screen that lets you see the initial screen if there isn't a Utilities menu where you can access Disk Utility.
At that point, you want to fire up Disk Utility and eject the backup disk (or start over without the backup drive connected) and erase the target drive.
This will wipe all data and the OS off that drive but it won't ask for the password since you aren't reading the data - just making a new blank filesystem on top of the old one.
Then you can proceed to boot and choose the blank drive to restore.
A full guide is available from Apple on how to erase and install an OS.
Just remember to not erase your only copy of bootable macOS or have a plan to make a bootable drive if you need one.