Here are a few whoppers that I've overheard other technical cofounders telling their non-technical cofounder:
Call me maybe. Right now it's really inexpensive to start on cloud platforms like Amazon AWS, Google, Microsoft Azure, Rackspace and a host of other cloud platforms. Self-hosting means your dev team / cofounder will be spending a chunk of time managing hardware instead of building new features.
This might be true for your startup. You might be the color of unicorn that happens to have a couple $50,000 servers laying around and just needs a data center.
But development time costs $1-$2/minute. Amazon's biggest, baddest instance costs around $5.50 per hour (it's a lot less if you commit to a [reserved instance])(http://aws.amazon.com/ec2/purchasing-options/reserved-instances/).
iOS is awesome. But it's like only selling to people who buy luxury cars. That's 11% of the market. Yes, there's more money there, but the rest of the market behaves differently. Usually the motivation for not wanting to go to market on other platforms is simple: when all you have is a hammer, everything looks like a nail.
This really means, I only know X, and I'm unwilling to learn anything else. Get a new cofounder, or you'll end up doing so later on when you have to migrate off X.