I think it depends on the faults you're talking about. If those faults in any way impact the business, then it's going to seriously affect how you view him as both a business partner and a friend.

In any case though, you're in a relationship with someone and communication is important. Your co-founder and you should be constantly providing each other feedback in order to be better partners. Be sensitive on how you word it when you bring it up, but if you stress that it's for the good of your friendship AND the business you've started, he should be able to take it in stride.