Vapor barrier on out building

My cousin is working on turning an old chicken coop into a functional space for working out/ playing music. Probably close to 100yr old building built with hollow clay block, concrete floor, wood framed roof with tin covering shake shingles. Not sure how he plans to heat/ cool the space, discussed mini split, but no solid decision made yet.

I plan on helping him frame the interior walls, so he can run electrical and then install some sort of wall covering soon. He's thinking shiplap or something, doesn't think any drywall. Out in the country and no inspections or permits, but trying to do mostly correctly.

The question is, what's the best way to vapor barrier this so he can hang bat insulation in the 2x4 wall cavities?

I suggested, 1/2" foam taped seems and spray foam the joints in hard to manage areas as I've seen that being method for basements. What about xx mil plastic sheets? Either stretched across back of 2x4 framing and stapled or attached to roof framing and draped to the floor, so the batting can't contact the block?

I'm not sure if the vapor barrier is essential as we're in South Central Kansas and heating/ cooling seasons are about equal here. Main goal is to protect the batting. But looking for input. We don't want to have to go back later.