My Finch has 5 egg but noticed today she has eaten one, the eggs are /- 15days old.?

The egg was not developing for what ever reason. Some birds will roll the undeveloped egg out of the nest, others will eat them so they do not attract predators in the wild. Calcium and protein are a commodity for birds so many of them will eat their eggs. Make sure that your birds have access to a mineral block in their cage if there is one but they do not eat it try moving it to a different place where it is more accessible for them. Good luck with the babies.