This sentence is compound and needs a comma after opened. (The fact that the first half and the second are of unequal length has nothing to do with how the sentence should be punctuated. Seriously, where do young people these days get their bizarre notions about how written English works?)
The doors finally opened, and when they did, we were met by a woman wearing combat fatigues and a deadly expression.
You could also add a comma before when, but with a relative clause (could be left out without changing the essential meaning of the sentence) immediately after a conjunction, the comma separating the relative clause from the conjunction is sometimes left out, too. Neither version is “more correct” than the other. The doors finally opened, and, when they did, we were met by a woman wearing combat fatigues and a deadly expression.