I have read that a missing or deformed tongue will make for poor food recognition and therefore poor feeding response, and the snake may even have difficulty striking, (this information was about ball pythons who have been brutally and mercilessly inbred in America all for the sake of 'some nice paint jobs' where deaths from fatal genes and deformities in clutches is so common it is expected and almost considered normal. It is needless to say that the ball python with the deformed tongue may well have had other issues going on, hence the difficulty striking) but this does not mean it will be so for your diamond.
How how old is it? You said this happened when it was young, indicating that it is no longer a "little one" as you put it. If it has managed to make it to an adult with this issue, then you should be okay, although it will more than likely always be difficult, if not more so than your other snakes.
Definitely get a refund. Demand one. What the breeder did was so dodgy and wrong.