Under WA state contract law if product was accepted and used is customer entitled to refund, offer acceptance WA contract law

This is a UCC Article 2 question.

Clearly, the buyer accepted the goods. For one thing, her guests ate them. I think it's safe to say that eating the cake is "inconsistent with the seller's ownership" of the cake. In any event, it's going to be awfully difficult for the buyer to return the cake.

Acceptance of goods doesn't, however, eliminate the buyer's remedy for their non-conformity -- at least if the buyer's letter few days later was "within a reasonable time" after she discovered the non-conformity. I suspect it would be.

The buyer's remedy is the loss she suffered as a result of receiving a white cake instead of a pale pink cake. Even allowing for the natural sympathy which a court might feel for a bride, I find it hard to imagine that the damages could possibly amount to half the price of the cake. While any prediction of the result of litigation is fraught with uncertainty, I would expect the bride would come out worse as a result of the litigation than she would if she'd kept the check for half the price.

Whether she should pay her off or forget it is, of course, another question.