Learn the lesson and move on.

Or rather talk to Person B and try tyo work something out (no threats!).

A lawsuit just sounds an expensive waste unless the company is pulling in a fortune. As far as I can make out you haven't put an awful lot into the site (sorry if that sounds harsh but speaking, perhaps, from B's point of view). He probably see's the code as the be-all-and-end-all of the business and so cant understand why you value your input so high in comparison (what percentage did he offer?)

I'd be inclined to forgoe lawsuits unless big money is on the table. And remember the mistake for next time :)