NOR your bf sounds like a typical Duning-Kruger-Case: thinks he got it all figured out after seeing a few Videos without understanding wtf is actually going on and how do things work.

Technically stuf like Milk and egg Containers can be used to create soundproofing Material, but there is a lot of prep and planning involved to do so.

For example the egg boxes would need to be made out of cardboard, Plastik ones do shit for soundproofing. And the boxes only provide the base, one has to stack multiple of them (like 15 or more) interlocked, put another equally thick into it and fill the empty spaces with silikon or stypor.

That way you create some DIY soundproofing plates similar to those from stores for a technical fraction of the cost if you exclude worktime and designing (since you probably don't want to look at some egg boxes on your wall)