[passionfruit]

Using an em dash would make the sentence clear.

"Through the window she saw George—a policeman—and several onlookers."

Or one could simply specify who George is by inserting "who is" after George.

"Through the window she saw George, who is a policeman, and several onlookers."

If using the Oxford comma does not make the sentence clear then the sentence needs to be rewritten.

[georgefox]

I agree completely, but the original sentence used a policeman as a member of the list, not an appositive. As an ardent lover of the Oxford comma, the only solution for this sentence is to move some words around.

[RyanMcGreal]

"Through the window she saw several onlookers and George, a police officer."

That fixes two problems with the sentence.