Hi Chris,
There isn't a way to get admin access to that source on your own. So if the ex-staff member isn't around anymore, there are a few options:
1. Make your own copy of the source, by uploading the same images, labelset, and annotations.
2. I can help you make a copy of the source, if downloading and/or reuploading the images yourself would be too onerous.
3. I can give you admin access to the source once I've learned/confirmed more about this situation. For example: is there a high chance the former staff member doesn't own their registered email address anymore; or is there another active team member who can vouch for the former staff member being not reasonably reachable.
If you prefer option 2 or 3 then we can go over more details by direct email; let me know.