Run this (as administrator):
Get-WindowsCapability -Online | Where-Object Name -like 'OpenSSH*'
Are both SSH client and server installed? Or does it look like this:
Name : OpenSSH.Client~~~~0.0.1.0 State : Installed Name : OpenSSH.Server~~~~0.0.1.0 State : NotPresent
If the SSH server isn't installed, add it:
Add-WindowsCapability -Online -Name OpenSSH.Server~~~~0.0.1.0
Upon rereading the question, I think you're right.
I wonder if he's using the tablets' IP address in the SSH command, or if he's using DNS (on his Wi-Fi/Cable) router, or if he's using an entry in the PCs' hosts file? If it's the hosts file, it could be that the tablet's acquired a new IP address from the router's' DHCP service and the hosts file needs updating.
That the reMarkable table can't be accessed vis SSH from Window and an Android tablet points to the reMarkable tablet (or the user using the wrong IP address).