#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.
"DIDDY! Diddy, what's the matter with you? Vernon? VERNON!"
Harry's uncle...
...In all the kerfuffle, nobody seemed to have noticed Harry...
2.) No inclination. Everyone was yelling at him, and Dudley had just blamed him for what happened. Why should he help him?
Harry's foot was on the bottommost stair wen Dudley found his voice.
"Him."...
..."BOY! COME HERE!"
3.) No proof. How do we know that Muggle chocolate would even help? See this question, 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
The school nurse had seen what Aunt Petunia's eyes - so sharp when it came to spotting fingerprints on her gleaming walls, and in observing the comings and goings of the neighbors - simply refused to see: that far from needing extra nourishment, Dudley had reached roughly the size and weight of a young killer whale.