At a minimum, the board member must refrain from participating in the discussion of any matter even remotely connected to the lawsuit, and should not be present during such discussions. Furthermore, the board member should have no contact with your attorney or any vendor who might be involved in the suit.

In practice, the board member should resign. The potential for a conflict of interest -- real or perceived -- is just too high, especially if the director is an officer. How are you going to convince the other shareholders that this director will be acting in the best interests of the coop if he's simultaneously so angry with the corporation that he's taking legal action against it?