I second this, it sounds cringey on the face of it. Maybe it works for non-technical customers, but I imagine technical customers would immediately be taken aback.

The only exceptions might be cases where very specific behaviors or implementation details are in question.

My main issue here is that we have a technical product, and the people we are selling to are developers.

If they could build it themselves, they wouldn't be talking to us, but my co-founder doesn't seem to get this.

It reminds me of this Silicon Valley scene - https://www.youtube.com/watch?v=JlwwVuSUUfc

Hmmm...The evidence is: he's made one sale. Any person can talk the talk. He's clearly doing things a salesperson shouldn't be. You, a non-salesperson know this. But perhaps he will shine through when the product is released. But to iterate, thus far, he has made one sale, that doesn't make you a salesperson. Yet.

He may be showing people the code because he's proud of it...

Speak to him.