Is the other system in the cluster being upgraded as well? Is the other system at another site or on the same site and just being used for resilancy?
I would ask more information why they need it removed from the cluster, as depending on your cluster set up it will have an impact. You shouldn't need to remove the system from the cluster to upgrade it, and if you remove it, then I would be very hesitant to try and put it back in again at a later date without any corruption occurring.