Send your client a thank you note
Express your gratitude for their vote of confidence. Your client takes the risk by sending a friend to you; honor it. Update them on the status of your conversation without breaching confidences. Let your client know that you spoke to the person that they sent and you have scheduled an appointment, or didn’t. Send them a thank you regardless of whether the referral went anywhere or not. My preference is to hand write a card and send by mail. People don’t do that very often anymore, so it is much more special than an e-mail. Do it the same day you speak with the referral.
Source: Stephen Wershing for SEI's "Practically Speaking" blog