re: How to deal with Unhappy customers...

The flowers on the 2nd cake are different in quality and far less realistic. They are the main visual difference to me… except for sizing of course.
She requested the difference in size and provided the flowers…. so I agree with MsMF, maybe you could of asked what else made her unhappy? It might have given you a clue as to whether she was genuinely disappointed or fishing for a refund.
There may be a subtle difference in the shine of the gold… but this could be a difference in lighting and photography. You cannot know exactly what product the original caker used to get the gold finish, and products vary greatly.