Does she have someplace to hide from the male? Perhaps she would do better in a cave or under a shelf instead of just lying on the bottom?
Cyanide was a major problem when I kept fish only tanks as a kid. It's really hard to say how long before they show signs of severe poisoning. Some fish could have been poisoned and never show any signs. They could go on to live a long time. Other fish would die within days or weeks of being introduced to your tank.
You can always tell a dead fish has been cyanide poisoned because it rots their liver. It turns their liver into jello. Leaves a gruesome soft, black spot on their side after they are dead.
Typical fish liver is located just above the pectoral fins and slightly behind the head. On top of where you would normally think of their guts. Usually somewhere around the lateral line, but it varies between species.
In my experience, a fish that is going to die from cyanide poisoning usually doesn't last long once they show signs of distress and start to act sick. They can be swimming along fine one day and then the next day, they look kinda sluggish. Couple hrs later (24hrs +/-) they are dead. But, thats just my experience.