If an ally attacks and defeats a minion with “Coordinated Attack” attached, I assume the minion and the…

Rules as written, if an ally attacks and defeats a minion with Coordinated Attack attached, any “When Defeated” abilities on the minion would resolve, then it and Coordinated Attack would be discarded, all of which occurs before consequential damage is dealt to the ally. Coordinated Attack has a constant ability reducing consequential damage which is no longer in effect when it’s discarded, so the ally would not have its consequential damage taken reduced.

This interaction wasn’t the intention when Coordinated Attack was created, and its ability should probably have created a lasting effect instead. We are fine with you playing it as intended, that an attack that defeated the attached minion still reduces consequential damage. However, we are not planning to officially errata this card’s text at this time, in part because it still functions under the current rules (reducing CD for attacks where the attached minion is not defeated).