[whamlastxmas]

Bring it up as a discussion topic, not a criticism. Ask how showing code to clients is beneficial if they don't understand or care, but in a more diplomatic way.

You could also ask him to develop in branches and give you time to look at it before pulling into master so you can get up to speed on changes and help catch any bugs. Offer to do the same yourself in reverse.

It also sounds like as the developer, you need to better at communicating what you're building and what it features. If your cofounder isn't even sure what all it does, you can be sure your customers don't.