User Deleted from our SS instance but cannot be added to other instances


I am part of a large agency group. One of my SS users left our organization and went to a sister agency. The sister agency also uses SS and cannot add this person--they receive an error that that this person is part of my SS instance. I've checked our user lists and they are not there. I'm not sure why they are getting this error and don't know what to do on my end. Any help would be great.