Your cofounder should be like your life partner in this situation. You should not have let it get to this point without knowing what was going on. Fix it now by

Your cofounder should be like your life partner in this situation. You should not have let it get to this point without knowing what was going on. Fix it now by leveraging your friendship to see how you can balance things out. If nothing else, show that your priority is to the friend. If it turns out his priority is not the same, well, lots of couples break up because one wanted kids and the other did not.