Hi Elodie
Thanks for posting in the Community!
If your trainee has left your organization but is still on your organization's Lucid account you can delete the user and transfer their files to someone else on the account (including yourself) as explained here: Delete a user and transfer their documents
If your trainee has already been removed from your organization's account but has shared their documents with you you can make a copy of these documents. The copies will show up in your account under "My Documents" and the owner of the new copy will be the current account (yourself). If the original document is shared with anyone else you will however need to reshare the copy.
Please let me know if you have any questions or need further guidance!