"She is not working overtime."
Translation:Hun jobber ikke overtid.
You don't have to worry about that, because they're pretty much always interchangeable. In Oslo where I am from we mostly use "jobber" for everyday speech, but "arbeider" is all right too. You can think about "arbeid" as "labour". If you'd say "heavy labour", it would be "tungt arbeid".
Ei* overtid? "hun arbeider ei overtid" lit. "she works an overtime." Why is the article included here?
In addition to being an indefinite article, "ei" moonlights as a negation, and it's the latter function it's filling in that sentence.
Beware that using "ikke" is vastly more common, with the exception of a few fixed expressions.