I working on the product what should my non-technical cofounder do?

Slowly establishing power and political structures designed to take advantage of you and the other technical staff while you are distracted building the product.

That way if/when they hit any big money it will be easier to maximize their own cut and push you out whenever technical integrity becomes an obstacle/irrelevant.

What needs to be done? If ya'll haven't discussed and agreed on the basics of that much, you're likely to need to real soon now. Write it down. When you have that, you can point to it and say "they should be doing the things on this plan that I'm not doing."

A "business plan" is a pile of bullshit you throw at people while begging for money; but you should definitely have a real one, somewhere. The list of things that need to be done for your business to succeed. Hopefully without reference to underpants gnomes.

Customer development and the non-technical parts of product development. Creating an actual business out of the product. Making sure that people want the product, understand it, willing to pay for it and so on.

There is no point building a product at all either before finding these out, unless it only takes a short time to build a minimum loveable product.

Talk to prospects, validate the need, market the product, write marketing copy, write marketing content, grow a lead list or a waiting list, create a pitch deck, write documentation etc etc.

There is a lot that can and should be done before building the product.

Anything and everything unrelated to development. User research? If you have an MVP then a ton of user testing

building a pre-order/waiting list