Hi Jules, sorry to hear about your father.
Are these external hard drives ? .. and are you trying to access them from a Windows PC ?
(Windows won't be able to read the Linux file system without some additional 3rd party software)
Probably the best way to access them would be to create a Linux LiveUSB, boot to it, and use that to copy off any files you want to keep to an external hard drive that has a Windows file system.
The other option might be to install one of these three pieces of software in Windows
https://www.howtogeek.com/112888/3-ways-to-access-your-linux-partitions-from-windows/that should allow Windows to read the Linux file system (supposing the drives are using the EXT4 file system, which is most likely).
If however you're able to boot into the installed Linux (using one of your fathers passwords), it would be pretty easy to copy off any user documents/pictures/music/etc. to an external hard drive (or USB stick) that can be read by Windows .. are you able to boot into Linux ?