To the best of my knowledge, there is no easy answer to this problem. The grounds for disqualification are mostly to do with non-attendance, criminal conviction or bankruptcy. So it is unlikely there is any means to remove the councillor who suffers diminished capacity.
There is, of course, an election for most councils next May, so it may be possible to persuade the councillor to retire at that point, or it may be possible to vote them off.
Apart from that, the best that can be done is for the chairman to make extra efforts to ensure the councillor is aware of what is going on at the point votes are taken, and to recap exactly what is the issue at stake.
Sorry I don't have any more substantial suggestions.