Witches be crazy sometimes.
Did you try offering her a tradeor cash for the egg, or to trade her a normal for normal, or a fire for fire when the egg hatches, so you can get back the genes of your late fire without her being out the free baby? I would try that, but I'd still be worried, because you said she wasn't prepared for any eggs or hatchlings, so even if she agreed to something she might foul up the incubation. Also, have you seen a picture of the clutch when it was laid? I'd be wondering if there were more than one good egg at the beginning, or even still, which could have guaranteed you a replacement fire with your late males genes, for the right price.