Remember that when a moderator is hired as a Stack Exchange employee, they have to step down as a volunteer moderator. The inactivity is irrelevant - a red herring - they weren't a mod during their employment, so they weren't an inactive moderator.
So, now that's clear, this is the sort of thing that's going to depend on the situation more than anything but the answer is kinda already laid out in the moderator reinstatement process.
There's not actually any reason for us to special case departing employees. If the employee wants to be reinstated, they can request reinstatement and we'll follow the process. It'll likely come down to just asking the mods on the site whether they have any concerns. Something similar has been done for others in the past and I don't have any reason to think we'd change it going forward.
Also, even if you still want to call them "inactive", there's nothing in the reinstatement process prohibiting mods removed for being inactive from requesting reinstatement and getting their diamonds back. So, again, the answer is the reinstatement process.
