I'll agree, Flash Player is not OSS (though projects to emulate this do exist), and it can be a royal pain but... You can definitely use Rails with Flex. Flex

I'll agree, Flash Player is not OSS (though projects to emulate this do exist), and it can be a royal pain but... You can definitely use Rails with Flex. Flex is not a server side language or framework. It can interface with all of them really well through web services. Heck, I'm pretty sure Flex can even inject and call JavaScript functions on your pages. Rails can render things to XML amazingly well, and you can still build an HTML response as well while maintaining the functionality of the Flex application. Your co-founder is comparing apples to oranges, but whatever works for you all is great. Just don't let the tools limit you.