Dumbledore says this before Mrs Figg enters the courtroom:
Dumbledore cleared his throat. The Wizengamot fell silent again.
"We do, in fact, have a witness to the presence of dementors in that alleyway," he said, "other than Dudley Dursley, I mean."
The notion of Dudley as a witness is thereby treated as self-evidently out of the question. But why should it be so?
- Are Muggles not permitted inside the Ministry? We've seen Hermione's parents in Diagon Alley.
- Are they not considered valid witnesses? Dumbledore later offers to summon Dobby to testify about the pudding incident, and house-elves are similarly "sub-wizard".
- Was Dudley unavailable? His school term still hadn't started, and even if it had, Dumbledore has ways of finagling things with the Muggle authorities.
- Was Dudley unwilling? Certainly most if not all of his previous encounters with the wizarding world have caused varying degrees of trauma, but judging from his behavior in Deathly Hallows, the fact that Harry had saved Dudley's own life and was at risk of losing everything he ever loved as a consequence might've been sufficient motivation.
- Were his parents unwilling? Certainly Vernon would've been hostile, but Petunia knows enough that she'd probably be open to persuasion -- "help the Ministry understand they need to crack down on the creatures that almost killed your son" or the like.
I'm tentatively leaning toward "invalid witness" -- elves may be considered subhuman, but they're at least part of wizarding society.