Translation:When my poor husband died, I would have gone crazy without you.
"mort" is both an adjective and the past participle of "mourir". In both cases it translates as "dead."
Quand il est mort [adjective] = When he is dead;
Quand Il est [aux. verb] mort [past participle] = When he died; When he has died.
So they look the same. In this case, you know it cannot be "When he is dead" because of what comes after: " je serais devenue folle/ I would have gone crazy," which is past conditional.
I think this verb tense is too difficult for Duo to teach because in American English, we are not so rigid in our phrase order. For example, we could put "without you" 1st, 2nd or 3rd in this sentence and it would be no big deal as long as you put the commas in the right place. Here, the wrong order is the wrong answer and you get dinged.
Is this how it works in France? In England, two men would be 'civil partners' and the term husband still only legally refers to 'husband & wife'. It's an interesting point though - I don't know what the legality is in France or if they have specific language for these circumstances?
Since 2013 (year of the legalization), every occurence of wife and husband has been replaced by spouses in our law texts.
For the @SueWaller comment, je could refer to a man (physically speaking), but we don't care. In this sentence, folle clearly states that the speaker is a woman, or at least female gender self-identified. So we need to keep devenue.