When his tongue started to grow rapidly.
Dudley ate a piece of toffee he found lying on the ground; however, that turned out not to be regular toffee, but rather magical candy that Fred and George made for their future shop. The candy made Dudley's tongue grow and he needed help to fix it. However, the Dursleys refused Mr. Weasley's help initially, but later changed their mind.
This is a scene from J. K. Rowling's Harry Potter.