This isn't a riddle, this is an ambiguous short story with many possible explanations. Riddles are supposed to hint to a single, correct answer. Also, they're generally not in the form of stories, although rarely they can be.
Riddles
Answer these riddles and the esophagus will let you pass.
Riddles for 'Lemmy'winks.
Time was set to 911 and not started is my guess.
Bingo
Your riddle states the microwave is on, defrosting a chicken dinner, meaning it's actively running. All modern microwaves count down from the entered cook time. This design is reinforced by the woman asking how much time is left on it. The riddle is poorly worded.
It would have made more sense if she turned out to be deathly allergic to chicken.
Yeah, if "the microwave was on and defrosting" had been left out, that would have been less misleading.
The best guess I can think of but it doesn’t feel right
The fact that he ran out before calling 911 makes me think the danger is a gas leak, but I’m not sure how the microwave would figure into it.
Maybe...
Tap for spoiler
They don't own a microwave. She was suffering from carbon monoxide poisoning so she was calling the oven, which was leaking gas, a microwave.
Nobody has ever solved this directly and it's likely OP did not post this looking for someone to provide the answer for them to confirm/deny (which explains why they haven't commented on the four comments that have been up for hours) but rather are looking for the answer themselves and are hoping more clever people will know it, even though their google searches have turned up nothing.
What a snarky reply
I suppose he misread something on the microwave display. But i dont use a microwave enough to know what it could say.
Perhaps they are vegetarians.