AOL seems to go its own way on more than a few style matters (example: US rather than U.S.), so the underlined just could be the way that news outlet handles the term. If one is bound by AP style, though, go with co-worker, not coworker.
The AP Stylebook says to retain the hyphen after co for nouns, adjectives and verbs indicating occupation or status. Other examples: co-pilot, co-author, co-star,co-host, co-owner.
In other combinations, AP says, lose the hyphen. Examples: coed, coexist, cooperate, coordinate.
Guess that makes Mr. Rewrite and Mrs. Rewrite codependent.