My co-founder will only sell if mvp is at same level of professional apps with huge UX UI teams [I will not promote]

Hi all, Having a bit of a rough patch with my cofounder, I am the technical cofounder who worked on the whole mvp to get it fully functional and a simple interface. As my other cofounder (who is also technical) didn't work on the app creation process, i gave him the marketing and sales tasks while I focus on priority bugs. He comes at me after our first customer saying the app is not usable because he didn't understand why we had a usability issue after ONE customer had a suggestion in UX UI. I told him MVP is function over cosmetics yet he seems to compare us with big corporations, seems he can't sell the app for what it set out to do for mvp. A salesman should be able to sell shoes to a snake, And he needs to sell the idea not the product as product have iterations on UX and also feature set. He only wants to sell if app is like a Ferrari of apps Any suggestions? [I WILL NOT PROMOTE]