A potential customer scheduled a call w/ our existing customer for a reference. Is there anything we should be doing with the existing customer to prep them / make sure they are good advocates or, is that likely to come off as shady?

I always directly tell customers to say nice things because it helps us and since they are an early customer helping us helps them. Customers understand that.

It would be good to prep your customers with questions that you can anticipate other customer would ask.  Just let them know your prospects was looking for good reference and you thought of them as a best fit. Most get it and stay positive. However,  do choose the people you want to expose.