Do you mean he has a membership separate from you and he made the reservation, or that you made a reservation for him using your membership?
If the former, you have two options: (a) transfer to him the points needed for the extra night and then he can modify his reservation to add it; or (b) you make the one night reservation in his name (names in your reservation must match his reservation exactly), and then you and he can contact MS and have it link the reservations, which will avoid having to change rooms.
If the latter, you can just modify the reservation using the online modifcation system to add the extra night.