If he would let you shoot the second buff, and it was on license/quota available, and the observer(male or Female) asked if they could instead, I would be inclined to, HOWEVER here are some issues to address. This person is potentially not practiced at shooting. The PH did not have them shoot when they arrived. Having a potentially inexperienced shot, take the shot could be risky for all involved.
While Buffalo quota may be left at the end of the season, that means a few big bulls walk, which just leaves more Bulls for the next year. If you are trying to keep a concession for the long term, having more big bulls is better. All outfitters are trying to make the most money they can, while still providing the best hunting, service, experience they can. Prices keep climbing it seems, most due to government increasing their fees, but the last few years have been pretty flat due to the economy, from my perspective.
In conclusion, I think being a returning client would give you the best chance to have something like this occur. With no preplanning and not sure if you plan to return, I would think it would be in the best interest of the outfitter to not allow the taking of the buffalo, maybe PG at no increased rate from observer. I would not pay the increase from observer to hunter, for the entire duration for the privilege to take a buffalo.