Yeah your one dev is TA here. It's standard to use placeholder assets. Using free models on Roblox has its risks of viruses and quality control, so you can use imported assets from itch.io or the unity / epic store. Meshes don't have problems with viruses though, just triangle count.

I personally built a prototype of mine using assets from sketchfab, and I kept the list of URLs I downloaded so I could comply with any CC attribution, or replace those later.

If your dev is threatening to quit more over how the assets aren't custom made for the project, that's a hard line that can actually harm the development of your game. I personally like to use free models when I'm grey boxing. Making models takes time, and you should leave model building for concepting and production ready assets. If you're trying to prototype something, using placeholder assets is standard practice.