Sched C = ongoing business activity.

Capital Property = keeping it to operate, such as a rental.

Sched D = one and done, not intended to do again, not ongoing, and more like, Hey, I bought a house to fix up and rent out or live in, but got such a great offer, that I just had to Sell it and take that gain on it.

You told us this:  "This house was not his main residence or second home. It was purchased solely to make some profits."

Intent seems to be Sched C, then. The improvement costs add to basis as COGS.