I was asked by my customer to build a Facebook game. The game would be something similar to Mafiawars.com where the game is hosted on a server and run through a frame on Facebook.

The thing is after several days of negotiations with the customer and near the finalization of the requirements he mentioned something strange. He said that if the game was successful on Facebook then we may add it to Google+ too.

I thought he meant that we'll develop a new version for Google+, but he refused as he argued that the game should be able to support both sites and he won't pay for the same game twice.

Now I haven't developed neither Facebook nor Google+ games before, so I don't know if it is possible to build a single Facebook/Google+ game.
How would you react to such requirement? How would you design such an application?

Notes

1. I confirmed with the customer that he wasn't talking about using Open ID he wanted full integration (sharing post, friend requests,..etc.)
2. I really don't want to lose that customer for numerous reasons (He even agreed to extend the project time to compensate for the time I need to learn Facebook/Google+ APIs)