Send them a link to a game of guess who and ask them how you are supposed to figure out the coworker without using descriptive words.
If they want to hard make your boss describe them to you instead of you describing them so it is their problem not yours
You: hey boss I worked with a person do you know who I'm talking about?
Boss: there are three people here.
You: tell me about them so I can tell you who it was.