Most often I would see a savvy recruiter pair up the lunch with the referrer so they could catch up with their friend and it would act as the 'sell' part of the interview slate.
This (lunch with referrer) happened to me when I was interviewing there and I agree that it is a smart move. It put me at ease. Presumably by the time you are there for an on-site, there aren't any questions of fit left.
This is a functional and smart practice.