Others have pointed out several reasons this isn't actually a problem (and that the premise itself is incorrect since "OpenAI" is not a charity), but one thing not mentioned: even if the MS-appointed board member is a MS employee, yes they will have a fiduciary duty to the organizations under the purview of the board, but unless they are also a board member of Microsoft (extraordinarily unlikely) they have no such fiduciary duty to Microsoft itself. So in the also unlikely scenario that there is a vote that conflicts with their Microsoft duties, and in the even more unlikely scenario that they don't abstain due to that conflict, they have a legal responsibility to err on the side of OpenAI and no legal responsibility to Microsoft. Seems like a pretty easy decision to make - and abstaining is the easiest unless it's a contentious 4-4 vote and there's pressure for them to choose a side.

But all that seems a lot more like an episode of Succession and less like real life to be honest.