#Lots of reasons. 

1.) **No opportunity**. As soon as he reached the house, the Dursleys started to make a big fuss over Dudley, not giving him an opportunity. 

2.) **No inclination**.  Everyone was yelling at him, and Dudley had just blamed him for what happened. Why should he help him? 

3.) **No proof**.  How do we know that Muggle chocolate would even help? See [this question](http://scifi.stackexchange.com/q/16495/33591), which makes a good case that it *wouldn't* help.

>Dementors are of the magical world; it would seem that to counter their effects would require some kind of magic (whether that's the regular chocolate, a magical ingredient, or both).

3.5.) How do we know that chocolate even works on **Muggles** to help with Dementor attacks? Perhaps it's effective only on wizards.

4.) **No need**. Dudley is going to end up eating chocolate anyway soon,  because that's what he does :-)

5.) Dudley is fat enough, he doesn't need more chocolate :-P