I always like to take stock of the who, what, when, where, why of an issue. That seems like a pretty sound method to use in this case too. Who is the person you want to mention, but didn't choose as a reference? What was the reason they left the company? When did they leave? Where does this fit into the context of the conversation you are having with the interviewer? Why is it relevant?
Work your way through some, if not all, of these questions and you should arrive at the answer that makes the most sense for the situation. The interview is about you, but networking and connections can potentially positively impact your situation.
Last comment, before you head off to have a great interview...You are there because your resume, experience, first meeting etc. made a good impression. Spend the time you have in the interview focusing on why you are going to be awesome in the job!