Must a wife re-fast a day in which her menses returned after it had ceased and she had performed ghusl, knowing that her monthly cycle ranges between seven and eight days?

Your wife's fast is valid according to the Maliki and Hanbali schools, and she does not have to make up the day because she completed it while pure. This is supported by what Ibn Qudamah mentioned in "Al-Sharh Al-Kabeer": "We have already mentioned that purity during menstruation is valid purity... So, if she sees blood for one day and purity for one day, she should combine the blood with the blood, and it becomes menstruation, and the period of purity between them is a state of purity, as we mentioned."