Does the current Board have to follow precedent and allow 6ft fences?
If only one homeowner had this approval (by mistake), then no, the current board not only does not have to follow "precedent" but is actually legally obliged to continue to enforce the covenants as written.
If the neighborhood had many fences in violation of the height covenant, then my response would be different, based on "abandoned covenant" principles.
should the current Board require the homeowner with the six ft fence to take it down?
No, because the owner at least in part relied on the board's prior approval when the owner spent the money to install the 6-foot fence. What the board should do is send the owner a letter saying that the board was wrong to approve the fence, and when the time comes to replace the fence (due to wear and tear or anything else), the new fence must comply with the covenants. The letter should also state that a copy of the letter will go in the lot's file and will be disclosed to potential buyers. The letter should be as polite and honest as possible. As needed the board should be clear that it is obligated to enforce the covenants and has no choice but to inform the owner of the situation and take this path to rectify the matter as fairly as possible to all.