Harry Potter question?

No. Muggles cannot see dementors. Dudley could sense the dementors.

From his point of view, Harry raised his wand threateningly (obviously not intending to do any harm but enjoying the effect of terrorising his cousin), them everything went dark and cold. He began to remember dark and horrible memories and his insides began to feel as though they were being filled by freezing water. He was then thrown to the floor by an invisible force, and then the next thing he knows, Harry is lifting the "spell" with his wand. I think to any muggle, it would seem as though it was Harry's fault.

Then of course when he gets home he blames Harry as that is what he honestly thought happened (although even if he knew it hadn't been Harry, he probably still would have accused him) and then, being true to his character, failed to believe Harry when he repeatedly demanded that he hadn't done anything

Yes, it means Dudley couldn't see the dementors in the movie.

Muggles cannot see dementors in the movies or books.

It's never explicitly stated anywhere in the films, but it's generally assumed that muggles cannot see dementors there either (they can't see other magical items like the Knight Bus either). Basically they can't see dementors.

For what it's worth, even if Dudley could see them, he would probably have blamed Harry regardless- the Dursleys tended to assume anything magical was Harry casting some evil spell or something to that effect. Even so, though, muggles still can't see dementors.

Dudley never saw the dementors, in the books or the movies.

No - Dudley couldn't see them.