Final answer:
The best course of action would be to tell her about the sale and encourage her to return next week to buy the dress.
Step-by-step explanation:
In this situation, the best course of action would be to tell her about the sale and encourage her to return next week to buy the dress. Offering a discount on the dress may not be necessary, as the customer may already be interested in the dress and simply needs a reminder to come back during the sale. Suggesting other dresses that are on sale could be seen as a distraction from the original dress the customer is interested in. Ignoring the customer is definitely not a good option, as it would create a negative impression and potentially lose a sale.