Play her at her own game. Go down and speak to the restaurant manager and explain everything. Then get the restaurant manager to ask his brother to telephone your 'friend' (using a disguised voice) and explain that the bill was overpaid by ?30 and that someone who perhaps didn't attend but paid might be due a refund. Then, date the restaurant manager's brother until he agrees to marry you. Ask your so-called friend to be your matron of honour. When you walk up the aisle in the groom turns around and reveals himself to be the restaurant manager's brother, you can then turn to your friend and scream 'now give me my thirty pieces of silver, Judas'. Believe you me, there'll be egg on her face for sure.